The Unapproachable Rainbow

I WAS amused and surprised by the hornet’s nest I stirred up when, last August, in the Atlantic Monthly, I took Thoreau to task for saying that he once stood in the abutment of a rainbow. The number of persons on both sides of the Atlantic, from whom I have received letters, who have stood in the abutment of a rainbow and seen the prismatic tints all around them; who have driven their cars through rainbows; who have played hide-and-go-seek with one; who have done almost everything with a rainbow except to snare it and bring it home with them, is truly amazing.

This is the passage in Walden, to which I referred: ‘Once it chanced that I stood in the very abutment of a rainbow’s arch which filled the lower stratum of the atmosphere, tingeing the grass and leaves around, and dazzling me as if I looked through colored crystals. It was a lake of rainbow light, in which for a short while I lived, like a dolphin. If it had lasted longer, it might have tinged my employment and life.’

The phenomenon which Thoreau goes on to relate, of seeing a halo of light around his shadow where it fell on the dewy grass in the morning, anyone may see under the same conditions, if he looks for it. I have seen it around my own shadow of a summer morning many times. There are no prismatic tints in it, however. But standing in the abutment of a rainbow, and seeing yourself standing there, is an impossible feat — just as impossible as it would be to climb the arch if you could reach it.

Just how Thoreau deluded himself, I am at a loss to know. We know that, as a rule, he was not an accurate observer, and that his imagination often colored his experiences. But a dozen or more readers rushed to Thoreau’s defense.

One man wrote me, at great length, contending that the rainbow was often elliptical in form, and that if the stormcloud was moving across the beholder’s front, the rainbow would move with it. ‘ Thus one end of the bow may be within a few hundred feet of the observer, while the other is a mile or more away.’ An elliptical bow, I suppose, is the result of the moving storm stretching or flattening the bow a little!

Another correspondent, a lawyer, writes: ‘ More than a dozen times have I as a boy kicked my bare foot into the very abutment of a rainbow’s arch.’ Then he goes on to say, ‘ Given a sultry afternoon with showers. The meadows are saturated with moisture. The air is laden with it. A quick outbursting sun behind you, and the one end of the rainbow will form at your feet. And you will see the green grass through a colored crystal.’ We all know that after a summer shower, when the sun comes out, many colored crystals may be seen flashing from the leaves and the grass. Every drop of water acts as a prism and dissolves the ray of light that falls upon it; but this is not a rainbow, and you get this effect only from a certain angle and from certain drops.

Of course, we do not all see the same things in nature, but we all see the rainbow under the same conditions, because the laws of optics do not change to suit our moods or conditions. No two persons ever see the same rainbow, because no two persons can ever use the same pair of eyes, and stand in exactly the same place at the same time. I would go a good way to see a rainbow travel, or move at all. It fades, but it is always motionless. The drops of rain and the storm-cloud travel, but the bow is stationary. Yet one of my correspondents, a woman in New York, saw a rainbow on its travels ! She first saw the usual double bow, but soon it and all the primary bow faded save about forty-five degrees of the latter. ’Within a few minutes this traveled northward [or from right to left] and I saw very distinctly the end of the rainbow resting on the barracks and on the sand. Its rate of progress was about two hundred and fifty feet per minute, and it passed within one hundred and fifty [What an eye she had for distance!] feet of the place where I was standing.’ As it moved, she said, the arc grew shorter, till it disappeared. Of course, the rainbow recedes from one with the receding storm, but it does not move from right to left, or across one’s front.

This correspondent, however, thinks it quite impossible to be in a rainbow visible to one’s own eyes, as Thoreau describes. Not so, however, another, a well-known poetess, who thinks she once stood in the abutment of a rainbow. She describes the prismatic colors on the leaves and grass after a shower, to which I have already alluded, and thinks she has clinched her argument when she avers that she once saw the ’foot’ of a rainbow resting on her own land a few rods away! Farther away than you imagine, my dear. I might be standing on a hill and see a rainbow spanning the valley below me, with one leg of it resting upon the highway, and see an automobile pass through it, but the occupants of the car would not see themselves going through the bow. An army officer in England, just home from the war, says he drove his car right through the foot of a rainbow. He must have torn a big hole in it, and endangered the whole superstructure; but he does not record that it fell and blocked the road.

No one has written me that he stood in the abutment of a rainbow yesterday or to-day. It is always on some occasion long past. In youth we do see and hear strange things. I often hear from persons who, when they were boys, have seen a snake swallow its young, but never from a grown person who has seen this. An Indiana Supreme Court judge wrote me to this effect recently. But the snake authority of Bronx Zoölogical Park, Professor Ditmars, I think, laughed him out of it.

One can easily settle the point about the rainbow by visiting a waterfall when the sun is shining, and when there is a volume of spray rising. Then he is pretty sure to see one leg of a rainbow, — the right leg, — which will not change its relative position with regard to him, no matter how much he moves. When he goes down-stream, it goes down; when he returns, it returns. It is a fragment of a bow which would form a complete arch exactly in front of him, were there a volume of spray sufficiently high and wide to meet the conditions; but he could no more get into it than he could filch its colors.

The phenomenon may be studied in the sprinkler on the lawn. I find that, when I walk by it, just beyond the reach of the spray, I can see one leg of a rainbow just large enough to show the curve. If I walk back, I presently see the other leg, but never the completed bow because the volume of spray does not extend high enough. The top of the arch of the bow, if completed, would be exactly on a line with me, and the sun behind me; I cannot see both legs from the same point of view, as we often do with the bow in the clouds.

So simple and easy seems the rainbow, like touching a button and seeing this marvelous apparition spring out! Yet it puts the natural philosophers on their mettle to explain it and analyze its laws. Its physics and its mathematics make the layman’s head swim. The raindrops have an outside and an inside, a convex and a concave surface, and both play their parts in the production of the bow. In the primary bow, the rays fall on the outside of the drops, and there are two reflections and one refraction. In the secondary bow, the rays fall on the inside of the drops and suffer two reflections and two refractions, which bring the colors in reverse order to those of the first. Then there is polarization, and there is interference, and there are other optical puzzles connected with the bow.

When the sun and the observer are on the same horizontal plane, as at sunset, the bow will be a half-circle. Earlier in the day it will be less than a half circle. It can never appear very near the zenith — from 42 to 54 degrees above the horizon is its usual position.

No rainbows are seen between the hours of 9 A.M. and 3 P.M., because the sun is too high in the heavens. The storm-cloud would need to be at your feet at such times. With your spray-pump you can produce a miniature rainbow at 10 and 11 A.M., but it will be on or near the ground. In Nature this never occurs. The lower the sun, the higher the rainbow in the sky, and vice versa. From mountaintops a bow making a complete circle has been seen in the valley below.

The physics and the mathematics of the rainbow are quite beyond most laymen. We all know, since our school days, that white light can be analyzed into the primary colors by the prism. Now every drop of rain has the effect of a prism, and splits the rays of the sun into the seven primary colors, and then the bow is born.

The only thing I have ever seen in connection with rainbows which was new and inexplicable to me, I witnessed one April morning while visiting a friend on the shores of the Maumee River at East Toledo, Ohio. The bank of the river, partly wooded, is about fifty feet high, and the distance across the river at that point about half a mile.

On a bright morning I was sauntering along the river-bank when, on looking across to the other shore, I saw two large prismatic patches, one above me and one below, and the sun squarely behind me. The day was clear, and there was not the least vapor or fog that I could detect on the surface of the water. The river was as smooth as glass. When I walked up the river, the rainbow-patches moved up with me, and when I went back, they went back. I could not get above or below them. I should say they were, approximately, about three hundred feet apart. They slowly increased in size till they became like long sections of a rainbow, reaching out to the opposite shore. When I last saw them, they must have been a good many feet in length.

I was called away to the other side of the house, and do not know just what happened in the interval; but about ten o’clock in the forenoon someone called to me that there was a rainbow in the river. And, lo and behold! there it was! — an effect I had never seen before — a perfect rainbow, with a span of several hundred feet, in the river. When I traveled up the river, it traveled up; and when I returned, it returned. It was, I think, a good many hundred feet away, and was altogether a strange apparition. I had to leave about that time, and do not know how long it lasted. My friends said they had never seen it before, and they write me that they have never seen it since.

There is another flitting and unapproachable apparition in the clouds, which everybody has seen, but which comparatively few have closely observed. I refer to the familiar spectacle of what country persons call ‘the sun drawing water.’ Great spokes of light radiate from rents in the clouds when the sun is partly hidden, producing an effect like a huge fan spread out in the eastern or western sky. The peculiarity of it is that the observer is always squarely in front of the middle of that fan; the vertical rays are always exactly in front of him, and the wings spread out equidistant on each side. You cannot, by walking or running or driving, change your relation to that phenomenon, any more than you can to the rainbow. Those vertical rays appear stationary, as, in fact, does the whole fan. Now, if someone writes and tells me that he has flanked that apparition and seen it off to the left or the right of him, I shall put him down as a nature faker.

What the explanation is of those spokes of light proceeding from those openings in the clouds, as if the sun were just there behind them, when it is so far away that its rays to human eyes would seem parallel with one another, I do not know.

Let the light come into your room through a crack or a knot-hole, and the rays do not diverge or spread out in that way. The explanation is probably a simple one, but I do not know what it is.