New Eyes on the Universe

I

PROFESSOR ALFRED NORTH WHITEHEAD has pointed out that, although science defines its subject matter as consisting of such things as are observed by the senses, — ‘ sights, sounds, touches, bodily feelings, shapes, distances, and their mutual relations,’— its actual performance is in sharp contradiction to this professed programme. Increasingly its researches push into domains beyond the senses. Its present frontiers are outside human experience. ‘ When Lord Rutherford at Cambridge knocks a molecule to pieces, he does not see a molecule or an electron. What he observes is a flash of light.’ When Dr. Hubble at Mount Wilson measures the red shift of the light from a remote spiral nebula, he does not actually see any reddening of the rays. Indeed, he does not see any fight at all, or any nebula. What he sees is a diminutive photograph of fuzzy lines — a photograph which required forty hours’ exposure to impress itself on the sensitive plate.

Without photography man would not be aware of these remote bodies. Not only is their light too faint for the eye to detect even with the largest telescope, but its more active rays are of a wave length to which the optic nerve is insensitive. When we survey the range of radiations which have been discovered in nature, and realize that in this wide sweep of sixty octaves of energy vibrations the human eye can discern only one octave, we are tempted to conclude that the eye was not fashioned to explore the universe. Science has progressed by augmenting eyes and extending eyesight with scores of devices capable of seeing the invisible, touching the infinitesimal, sensing the impalpable. Practically all our knowledge of the world within the atom and beyond the stars has been procured in indirect ways.

These indirect ways are dependent on eyesight to get their message delivered. The researcher must use human vision to read his instruments. To that extent — and it is fundamental — the senses remain the ultimate arbiter of knowledge. It is clear, though, that the observer has no direct knowledge of the invisible entity he is studying, but only of its effects on visible objects of the apparent world — such things as photographic plates and fluorescent screens, meter pointers, and the like.

The modern scientist is like a detective who finds clues, but never gets a glimpse of the fugitive he seeks. From thumb prints, foot tracks, handwriting, and other circumstantial evidence, the detective builds up a picture of what his quarry must be like. Similarly the physicist builds his picture of the atom from clues which the atom lets drop, or from other clues which the researcher twists or tortures out of its invisibility by ingenious tricks — indirect sights, indirect touches.

Much of our present knowledge of atomic structure, for example, has come from the practice of bombarding atoms with particles and measuring the angles at which the particles rebound. The feat is comparable to that of throwing tennis balls against some unknown object in the night, and from the directions in which the balls bounce back and the energy of their recoil determining the size and shape of the object, and finally concluding that it is a log cabin. This may seem to be a curiously indirect and cumbersome way of measuring log cabins, but if the cabin is a mysterious structure which cannot be seen by mortal eyes or felt by mortal hands, though fortunately it docs reflect tennis balls whose resultant behavior we can measure, who can object to the method so long as it yields trustworthy results? In a certain sense the bounced balls are an extension of the hand that throws them, and in that sense we may say that we do touch the cabin — or the atom. ‘ I claim,’ said Professor Gilbert N. Lewis, ‘ that my eye touches a star as truly as my finger touches this table.’

II

In this sense, the unaided eye of man can ‘touch’ about six thousand stars. That is the limit of human vision. When Galileo made his ‘optick tube’ and through its 21/4-inch lens turned his gaze upon the heavens, — first of mortals to behold that glittering expansion ! — he was surprised to find that he could count ten times as many stars as he could see without the telescope. Perhaps he had expected the glass to magnify the stars, as it had enlarged the image of distant ships outside the harbor at Padua. But no, the stars remained mere points of light — the only difference being that through the telescope they shone more brightly, and fainter orbs invisible to the unaided eye were brought to view. If he built a larger telescope, would it reveal yet more stars? And might not some of them be so magnified as to show a measurable disk like the sun’s?

Galileo built larger and yet larger telescopes, and his successors have kept up the pace, until now we have one with an aperture of one hundred inches diameter, the great Hooker reflector at Mount Wilson Observatory in California. Its light-gathering power is equivalent to that of ninety thousand human eyes. Thus it will render visible objects which are ninety thousand times fainter than the faintest of the six thousand that overawed the Psalmist. Indeed, this 90,000-eyed Argus has brought more than five hundred million stars to view, and by photography has revealed the existence of as many more again, so that to-day man has the instrumental means of studying a thousand million stars. But not one of these distant suns, neither the seen nor the photographed, reveals itself in the image of a disk. The stars are too far away for such intimacy. All remain, even under our highest magnification, points of light.

Astronomers, chafing against such limitations, calculated the optical requirements. Their figures showed that a telescope would need a lens or mirror of twenty feet diameter in order to show the beginning of a disk of even the largest star. The possibility of attaining so prodigious an optical piece seemed very remote indeed. Since 1929 a staff of experts has been planning and working to build a 200-inch reflector, which is assumed to represent about the upper limit of manufacturing possibilities under present conditions. The time that has been consumed, and the innumerable labors of testing that have been involved in even the preliminary work, suggest the magnitude of the undertaking. With the attainment of new materials and the improvement of technique, even larger instruments may eventually be financed, constructed, and installed; but meanwhile there was only the 100-inch, and this consuming desire to focus the giant Betelgeuse under an eyepiece and see its image as a red disk — something to be measured as the microscopist maps the dimensions of a blood corpuscle.

If the result could not be attained directly, possibly it might be got indirectly — like the measurement of the log cabin in the dark. If it was impossible to produce the image as a disk, perhaps one could manipulate the light rays from Betelgeuse in such wise as to make them betray some of the unmistakable characteristics of a disk, and from these clues infer what the image itself would be. In other words, look for thumb prints, footprints, handwriting, and other characteristics.

So it has worked out. An ingenious apparatus devised by an American physicist, Dr. Albert A. Michelson, provided the astronomers with an instrument which makes it possible to read the meaning of the starlight’s behavior in terms of the diameter of the star. The interference bands, those fuzzy rings which surround the image of a point of light seen through an opera glass or telescope, are the thumb prints, footprints, and handwriting which become so singularly significant to the interferometer. It translates them into angular dimensions, and, by an actual sliding of two oppositely positioned mirrors on a twenty-foot horizontal bar, the numerical index to the star’s diameter is indicated. Thus, from certain fringes of its light, the giant Betelgeuse was wheedled into revealing its girth — which turns out to be that of a diameter of two hundred and fifteen million miles. The device has since been used to measure five other stars. One of them, the red Antares of the Scorpion, shows the enormous diameter of four hundred million miles — a diameter more than twice that of the earth’s orbit around the sun.

To tape smaller stars a wider separation of the two mirrors is necessary, and at Mount Wilson a fiftyfoot interferometer has recently been constructed. With this instrument — which, for its special kind of work, is equivalent to a telescope of one hundred feet diameter — it is possible to reach more distant and smaller stars, perhaps to get down into the sun’s class of dwarf stars, possibly even to mensurate the rarer smaller white dwarfs that here and there betray their presence and their peculiarities in the telltale lines of their spectra.

III

Stellar spectra are the most revealing of all the celestial clues that come to us. Perhaps the greatest physical experiment — if one may judge by its fruits to this day — was Sir Isaac Newton’s investigation of ‘the celebrated phenomena of colors.’ He cut a small round hole in his landlady’s window shade, passed a beam of sunlight from it through a glass prism, and discovered the nature of the solar spectrum — a mixture which the triangular rod of glass sorted out into the familiar rainbow pattern. Indeed, the rainbow itself is a huge solar spectrum set up in the firmament, as though to give man a broad hint of the master key that lay waiting all ready to his use. How long it had to wait! In the early nineteenth century, a hundred years after Newton, Auguste Comte predicted that man’s curiosity must forever be thwarted by the heavenly bodies — for, said Comte, man can never know the composition of a star. He reckoned without the rainbow — but so science, too, reckoned ignorantly for many groping years.

The thing was not entirely neglected, though. There was a young Scot, Thomas Melvill, who thought to direct the prism toward firelight and see what it would show. In the flames of burning alcohol he successively introduced particles of alum, nitre, potash, sea salt, and was fascinated by a bright yellow spot that flared up brilliantly at each burning — and always in the same position in the spectrum. That ubiquitous yellow spot was a tantalizing mystery for a hundred years.

Meanwhile, it occurred to another Englishman, William Wollaston, to introduce a change in Newton’s original set-up. In place of a round hole that had been used to admit the light in all previous experiments, Wollaston substituted a narrow slit. When sunlight passed to the prism through this slender opening, it was broken into the familiar colors as before, but Wollaston saw something that no other observer had ever beheld. The spectrum was sharper than the shaggy rainbow of previous observations, but a more remarkable difference lay in this: it was traversed by seven dark lines. They were curious — seven parallel slits of shadow cutting athwart the seven broad bands of color as though to separate one hue from the next. Wollaston accepted this as the explanation, and with that easy guess dismissed the subject. Seldom has a man missed fame by so slight a margin. He had glimpsed the key to the stars, — the key to the atoms, as it has turned out, — but did not know it.

Destiny was not idle. Over in Germany a train of events was shaping. It began in Munich with the collapse of two old dwellings in a wretched alley, crushing their occupants in the ruins. Rescuers searching the debris found one survivor, a fourteen-year-old orphan, Joseph Fraunhofer. It chanced that the Elector Maximilian Joseph witnessed the rescue. Appalled by the tragedy, and impressed by the providential deliverance of the boy, he bestowed on young Joseph a purse of eighteen gold ducats.

With part of his gold the astonished boy bought freedom from the tradesman to whom he was apprenticed. The remainder went to buy books and tools with which presently he set up shop as a polisher of optical glass — meanwhile burning the midnight candle in rapt study of mathematics and optics. Eventually Fraunhofer was known all over Europe as ‘the great telescopemaker,’ a title won by designing and shaping ‘the great Dorpat refractor’ for the Russian observatory. Its lens of nine and a half inches diameter was for years the largest telescopic glass on earth — and the most powerful, as was attested by many remarkable discoveries.

But the greater discovery is not any of the stars that were sighted first through the great Dorpat refractor, but a quite incidental find that Fraunhofer made in the course of testing combinations and curvatures for its lens. He was striving to attain perfect light transmission, and this led him to study the composition of sunlight. He made a large prism of exceptional clarity, and, in order to intensify the sunlight, passed a narrow beam through a small telescope and focused it on the prism. The result was a broad spectrum of brilliant colors traversed by thousands of dark lines. Wollaston, twelve years before, had reported seven lines, but Fraunhofer counted six hundred, and saw many more too fine for sharp definition.

It should be noted that this was an original and independent discovery, as many other rediscoveries in science have been. The telescope-maker knew nothing of the British physicist’s work, and he was vividly excited by the novelty of these strange parallel shadows that persistently marred his rainbow. He shifted the apparatus to different positions, but there was no change in the dark lines. He tried early morning sunlight, noon light, that of afternoon — but time made no difference in the position of the lines. They were inherent in the light, and, presumably, in the source of the light.

Fraunhofer set to mapping the lines, and carefully charted the positions of 324. The more conspicuous ones he took as landmarks, sighted their positions precisely through a theodolite, and labeled each with a letter of the alphabet. Two dark lines in the yellow part of the spectrum were designated with the letter D, for example. This early nomenclature remains to the present day, though with thousands of extensions.

The investigator next turned his prismatic eye on the moon and the planets, and found in their light a fainter replica of the sun’s spectrum, as would be expected of reflected sunlight. He tried the stars. Castor showed a markedly different pattern; instead of many fine lines, he could distinguish only three broad bars of shadow, two in the blue region, one in the green. The spectrum of Pollux was quite different, somewhat similar to the sun’s, while that of the bright Capcila was almost a duplicate, in dimmer outline, of the solar pattern.

Then, in examining the light of laboratory flames, Fraunhofer made still another discovery. That bright yellow line which had impressed everybody who had ever looked at a flame spectrum showed up through his more powerful prism as two yellow lines. He looked for them in the solar spectrum and found their counterparts in two dark lines, his D lines! Obviously there was some sort of inner harmony here — but a telescope-maker cannot continue indefinitely in bypaths, no matter how fascinating. Fraunhofer published his findings in a German technical journal, and turned back to his main job of fashioning the most nearly perfect telescope lens in the world. When he died eleven years later, in 1826, aged thirty-nine, two words were cut on his tomb: Approximavit sidera. But science was to puzzle another twenty-five years over this curious complex of dark lines by which he had indeed reached the stars.

IV

By the middle of the nineteenth century it was generally accepted that colors were an index to the kinds of substances that were making the light. This seemed indicated by all tire experiments, and was conclusively demonstrated by two Heidelberg professors, Gustav Kirchhoff and Robert Bunsen. The two bright yellow lines, for example, were identified with sodium; their universal presence in laboratory flames was accounted for by the fact that sea salt (which is half sodium) is diffused through the atmosphere in minute floating particles, and so inevitably enters all flames. But if the same element was burning in the sun, why did it show in sunlight as dark lines ?

Kirchhoff found the answer in a simple experiment. He vaporized sodium in his laboratory, and directed a beam of sunlight through the hot vapor and on to his prism. In the resulting spectrum the dark D lines showed thicker, as though their shadows were deepened by the vapor. Would the gas have a similar effect on a flame spectrum ? Kirchhoff lighted a flame which gave the familiar yellow sodium lines, shot a beam of its light through the glowing vapor of sodium, and instantly the bright yellow lines became dark D lines — identical with those of the solar spectrum.

So that was the secret. Sodium vapor in the sun’s atmosphere absorbed the light generated by incandescent sodium in the sun’s body, thus leaving dark blanks in place of the absorbed rays. A test of other metals in the laboratory confirmed this. Kirchhoff tried iron, and found that iron vapor absorbed its rays and left corresponding dark lines. He tried magnesium, calcium, copper — and in each case the result was the same. An atom, it seems, is somewhat analogous to a tuning fork. If it is capable of generating vibrations, or rays, of a certain wave length, it may also absorb vibrations, or rays, of the same wave length — given the right gaseous conditions.

Kirchhoff’s experiments not only cleared up the meaning of the Fraunhofer lines, but also pointed out how they might be used. For when it was recognized that the dark lines are the colored lines in blank, and that each signals the presence of a certain element in a certain energy state, it was necessary only to match terrestrial spectra with stellar spectra to determine the composition of the stars. The same iron lines that flared in the laboratory flames showed in the solar flames — and so with each of the elements. There was a language common to earth and the stars, and man at last had learned it.

And what a packed language the spectrum is — how versatile its expression, how economical its code! From these lines the astrophysicist has learned not only the composition of the stars, the kinds, quantities, and thermal state of their substances, but also such varied details as the stellar luminosities, distances, directions of rotation, motions of approach or recession, and, more indirectly, their sizes and masses. It is not strange, therefore, that many of the large telescopes are auxiliaries to spectroscopes — that is, their main work is to feed light into a train of prisms.

As telescopes have grown in size and in light-gathering power, so too have spectroscopes lengthened their focus and enlarged their prisms. At Mount Wilson there is a huge instrument with prisms eight inches thick, beautiful solid blocks of transparency which weigh, with their mountings, one hundred pounds each. The spectroscope combines two of these large prisms with a mirror to reflect the light back through them, thus gaining the dispersion effect of four prisms. With this a spectrum of twenty inches length may be obtained, whereas the best star spectrum of twenty years ago was less than four inches. The longer the spectrum, the wider the separation between lines, and the greater the number of distinguishable lines.

In the physics laboratory the spectroscope has become as indispensable as in the astronomical observatory, for these dark and bright lines are the thumb prints of the atoms, and provide many of the data on which modern atomic theory has built its amazing picture of the infinitesimal. Theoretically, each element may generate thousands of lines peculiar to itself. Several thousand lines individual to the iron atom in its various stages of excitation have been identified in the sun, for example, and altogether more than 20,000 spectral lines in the sun have been mapped. Of all spectra, solar, stellar, and those produced in the laboratory, more than 200,000 individual lines are known.

For bright light sources — such as the sun, laboratory flames, arcs, sparks, and glows — it has been found desirable to substitute for the prism a diffraction grating. This is a flat plane of hard metal which has been minutely furrowed with a series of delicate parallel lines cut into its surface, sometimes as many as 30,000 to the inch. When a beam of light strikes this surface, its rays are diffracted by the tiny furrows, sorted out into their progressive order of wave lengths, and a spectrum of high magnification and fine detail is the result. At the Massachusetts Institute of Technology a spectroscope of the grating type recently installed occupies a whole room, and throws on a semicircular screen a spectrum of forty feet length. Under this wide dispersion, lines that ordinarily show up as single lines separate into doublets, triplets, or even more, and the intricate complexity of the atomic gyrations which generate light is suggested.

V

The spectroscope is a sorting machine; the photographic camera is a recording machine; when the two are combined they make a spectrograph — that, is, a spectroscope which photographs its rainbow. Most of the spectroscopes used in scientific research are spectrographs. Obviously there is an advantage in having a permanent record which may be studied at will and filed for reference. But the spectrograph has yet more fundamental advantages. It records fine lines that the unaided eye cannot see because of the low intensity of the rays. Also, the spectrograph is sensitive to vibrations in the ultra-violet and the infra-red, radiations to which the optic nerve is numb.

An example of these advantages in action may be taken from a bit of current research. For long years solar physicists have been sifting the sun into its elements. Beginning in 1859, when Kirchhoff and Bunsen discovered the presence there of iron, sodium, and others of its more conspicuous ingredients, the analysis has advanced, clement by element, as spectroscopes became more adept at resolving faint lines and as photography extended its sensitivity farther into the invisible. By the beginning of 1934, fifty-nine chemical elements had thus been identified in the solar fires. But the earth shows ninety-two elements. Science, therefore, has been searching the sun’s rays for the missing thirty-three that will round out its identification of earth stuff with sun stuff.

Recent gains in photographic sensitivity have been in the range of long wave lengths that extend from the red rays into the heat rays. Thirty years ago it was difficult to photograph red light; in that region of the visible, the eye could see more than the camera; and photography was of little use to an investigator of red phenomena. Today it is possible to photograph not only the whole gamut of red, but also far out into the invisible infra-red.

During 1932, a physicist at the Bureau of Standards in Washington, C. C. Kiess, was making a spectrographic study of various elements with the aid of the new infra-red photography. He found that the light of burning phosphorus gave a number of lines far out in the infra-red region. It was already known that phosphorus shows several lines in the ultra-violet, and none at all in the visible spectrum. That is to say, no human eye has ever seen the light of phosphorus. When you strike a match, it is the flames of sulphur, oxygen, and other participants in the combustion that flare into fire; the most active agent, the phosphorus, gives off only invisible heat rays and invisible ultra-violet rays.

Now, to get back to the sun. In 1934, at the Princeton University Observatory, a research assistant, Charlotte E. Moore, began a search among lines measured on some solar spectrograms made at Mount Wilson. The plates had been photographed there by H. D. Babcock with the great solar spectrograph, using the most sensitive infra-red plates available, and many lines far out in that invisible region of radiation were rendered visible. Might not some of the new-found solar lines signal the presence of phosphorus? Dr. Moore pursued that question. Of the phosphorus lines observed by Kiess in the laboratory, she selected those which should be present in the solar spectrum if the element were detectable by this means, and compared them with the infra-red solar lines photographed by Babcock. The three leading lines agreed with reasonable accuracy, and two fainter ones seemed in probable agreement. Many careful measurements of position and intensity were made before the conclusion was drawn. At last, in April of 1934, the discoverers announced in a brief note in Science that ‘phosphorus may now with considerable assurance be added to the list of elements present in the sun.’

The spectroscope alone could not reveal that presence. Ordinary photography, though it is most sensitive in the ultra-violet region where there are strong phosphorus lines, could never have detected it — because, curiously, the extreme ultra-violet rays of sunlight are absorbed in the upper atmosphere, including those of solar phosphorus. The infra-red rays have no trouble getting through, — they are among the most powerful of the radiations that reach us from the sun, — but until recently there was no way of photographing the extreme infra-red. Since 1924 the photographic reach into this invisibility has increased four fold.

Not only has photography steadily pushed its conquest into unseen realms, but even in the visible region it has shown an advantage which the eye does not possess. This is its ability to build up an image with time. The eye either sees or it does not see. But, with a photographic emulsion, what may not be imaged in one second or one minute may be clearly got if the exposure be prolonged many minutes or hours. This fact was first recognized by George P. Bond, then director of the Harvard College Observatory — where the first photograph ever taken of a star was made in 1850, and where stellar photography may be said to have been cradled. As early as 1857, Bond pointed out that, when a photographic plate is exposed, a certain time elapses before any observable image is formed. The length of this period depends on the intensity of the star’s light and other conditions, and provides a precise way of measuring brightness. Instead of the vague visual estimates then current in astronomy, — by which the twenty brightest stars were ranked as of the first magnitude, others not quite so bright as of the second, and so on, — Bond proposed that stars be rated according to the length of time required to photograph them. Out of this suggestion has grown the modem science of stellar photometry.

Tests at the Kodak Research Laboratories show that from four to perhaps one thousand quanta (or units of light) are necessary to render a single grain of photographic emulsion developable — the lower limit, four quanta, producing a barely perceptible deposit on a high-speed plate. The rods and cones of the retina are the equivalents in the eye of the silver halide grains in the photographic emulsion, and recent research shows that one quantum is sufficient to activate a rod or a cone. It is difficult to make a scientific comparison between the eye and the photographic plate without overburdening the statement with technical qualifications, but in general the idea may be suggested as follows. If we take light of the wave length 5550, which is in the green region of the spectrum where the eye is most sensitive under normal conditions, and compare the intensity of illumination required for seeing (in a half-second exposure) with the intensity of illumination required for photographing, we arrive at this interesting comparison: for minimum seeing, 20,000 quanta per square centimetre; for photographing, 30,000,000,000 quanta per square centimetre.

On this comparison, the retina is at least a million times more sensitive than the photographic plate — though, it must be admitted, the wave length used in the foregoing example is not in the region at which photography is most effective. In that most effective region, the photographic plate requires only about a tenth as many quanta, or 3,000,000,000 per square centimetre — which leaves the eye still thousands of times more responsive to light than the most sensitive photographic emulsion.

But the eye is limited by the ‘now’ factor. It is the number of quanta that strikes the retina at the moment, within the allowed half second, that determines whether the image is seen or not. The photographic emulsion is not so dependent on the instant: it responds to the cumulative effect. The faintest objects that have been photographed with the 100-inch telescope are nebulæ at a distance of more than 300 million light-years; their light is so scant that on the average only one quantum per second falls on each twenty square inches of the mirror, or a total of 500 quanta per second for the entire mirror; yet by continuing the exposure through several nights the necessary millions of quanta are gathered and the image slowly accumulates. The inanimate grains of the emulsion are permanent ly altered by the absorption of the light units, whereas the animate elements of the eye continually readjust their molecular mechanism for the next impact, and so hold no alteration as permanent, no arrangement as the beginning of an image later to be filled in.

The faintest stars that can be seen with the unaided eye are of the sixth magnitude — equivalent to the light of a candle seen at a distance of six miles.

When the eye looks through the 100-inch telescope, which multiplies the eye’s light-gathering power by ninety thousand times, it can see stars down to the nineteenth magnitude — a candle at a distance of 2400 miles.

But when the photographic camera is clamped to the eyepiece of the 100inch telescope and exposed for long periods, stars of the twenty-second magnitude show up as dim images on the most sensitive plates — a candle photographed with the same apparatus at a distance of 9600 miles would show the same brightness.

This is about the limit of present telescopic photography, but when the 200-inch telescope is installed it should be able to photograph objects down to the twenty-fourth magnitude — a luminosity equivalent to that of a candle viewed through 24,000 miles of space.

VI

The remarkable advances recently made in the technique of telescope design and construction have already been outlined for Atlantic readers by Dr. George Ellery Hale, the master builder. Almost as remarkable, if not equally so, have been improvements in auxiliary devices which extend the reach or multiply the strength of the telescopic eye. An example, among several that might be cited, is the photoelectric cell.

This device—the so-called ‘electric eye’ of television, the sound pictures, and hundreds of other industrial applications — has long been used in scientific research as a detector of radiation and a measurer of its intensity. For several years Dr. Jakob Kunz, of the University of Illinois, has been producing cells of an ultra-sensitive type for the use of astronomers in gauging starlight. Quite recently Dr. A. E. Whitford at Washburn Observatory, Wisconsin, introduced some important changes in the mounting of the apparatus; he installed the cell and its amplifying tube in a compartment from which the air is exhausted, and produced a stellar photometer of extraordinary responsiveness. It has been used at Mount Wilson by Dr. Joel Stebbins and Dr. Whitford to measure faint luminosities beyond the grasp of photography — for example, invisible outlying stars of the Andromeda Nebula. Photographic surveys have measured this outside galaxy as of about 40,000 lightyears diameter, but the electric eye sees it as double that, and therefore of dimensions comparable to the scale of our own Milky Way. This electric vision conveys its report by means of a spot of light moving along a graduated scale — indirect vision, like the tennis balls bouncing off the log cabin, but accurate vision nevertheless.

It is in these auxiliary devices that the next great advances in astronomical equipment seem promised. In 1934 the Harvard College Observatory installed at its new station on Oak Ridge, Massachusetts, a 60-inch parabolic mirror of .3 to 1 focal ratio. That is to say, the mirror is so deeply concave that the rays collected by its 60-inch reflector are focused at a point only 18 inches from the mirror. This focus is too short for a definable image. It will not help eyesight or photography, but it provides an effective trap for collecting light and feeding it into a photoelectric cell, thermopile, or other sensitometer. It suggests a type of observation that will be increasingly developed during the next decade.

Our new eyes of the universe are less subjective than the old, and more objective — and that, we believe, is a gain. The universe, as J. B. S. Haldane has suggested, may be not only ‘ queerer than we suppose, but queerer than we can suppose.’ For the exploration of that sort of universe we need more than human eyes.