Art. LXI.—On some Peculiar Cases of the Reflection of Light

Transactions and Proceedings of the Royal Society of New Zealand, Volume 27, 1894, Page 535

Art. LXI.—On some Peculiar Cases of the Reflection of Light By Major-General Schaw, C.B., R.E. [Read before the Wellington Philosophical Society, 17th October, 1894.] The reflection of light from smooth water or water agitated by gentle ripples is a natural phenomenon which is familiar to to us all, whether the light be that of the sun, or moon, or a brilliant planet like Jupiter, or of artificial lights like those in a harbour or on board ships; but in all these cases we see the light as well as its reflection doubling it in the water. There are cases, however, in which we observe only the reflection, the source of the light which is reflected being hidden from us; and such cases are at first somewhat puzzling. One such case I observed about this time last year, and I have seen it again this year during calm bright weather a little before sunset. It is only in spring and autumn that the phenomenon is observable from my point of view, although doubtless it would be seen at other times of the year, when the sun sets farther north or south, from other points of view. The window of my sitting-room in Aurora Terrace is about 180ft. above the sea, and it faces towards Clyde Quay, or south-east. When the sun is lowering towards the west, and is not visible from my window, a singular appearance of reflected light is often to be seen on the comparatively calm surface of the water towards the Te Aro Baths; and at first I was unable to trace, the source or sources whence the reflection came. None of the windows of the houses on the hill-side above the water showed any bright reflection of the sun; nevertheless the reflection was due to some of these windows, as appeared evident on consideration, and as will be seen from the annexed diagrams (Plate XLVI.). First, let us look at the course of the sun's rays as seen in section, or looked at sideways. At the time when the bright reflected patch of light was visible on the water at W from the point of observation at O, the sun was at such a height that the ray of light in the direction S H was reflected downwards to the water at W, and thence again upwards to the eye at O; the angles of incidence and reflection in both cases being of course equal. The reflection at W was repeated many times on various inclined surfaces of minute wavelets, and so produced a patch of sparkling light of some size, considerably larger than the window whence the light was reflected. The reflection of the sun from the window was not

directly visible at O, because the rays were thrown downwards to W, not in the direction towards O. The bright patch of light persisted for some minutes, notwithstanding the revolution of the earth, which causes the apparent downward movement of the setting sun, both on account of a slight dispersion of the rays reflected from the window-panes, and also and more specially on account of the varying inclinations of the reflecting surfaces of the wavelets. But not only is it necessary that the eye should be at the right level to see the reflection, the sun also being at the right elevation to produce it, but also the position of the eye in plan with reference to the direction of the sun's rays, and the orientation of the wall of the house, a window in which acts as the first reflecting mirror. This will be seen from the plan showing the course of the sun's rays if seen from above. The position of the setting sun in spring and autumn is about as shown on the plan, and it will be evident that the window acting as the first reflector must be such that the angle it forms with the impinging ray is equal to that formed by the reflected ray which reaches O in consequence of its second reflection from the surface of the water. If the building were in any other alignment the reflected ray would be otherwise directed, and would not be seen at O. It will be evident, also, why I only see this reflection at this time of the year or in the autumn. When the sun sets farther south in the summer, or when it sets farther north in the winter, the reflections are thrown towards the north or the south, and are not visible at O, as they are when the sun is setting in the intermediate point. But by shifting the point of observation the reflection no doubt would be seen when the sun is at the same elevation, if the point of observation were at the same level, or, rather, in the same inclined plane. The effect is the inevitable result of the law of reflection of light from reflecting surfaces—viz., that the angles of incidence and reflection are always equal and in opposite directions; but, to see the effect, the eye, the vertical mirror (which is the window), and the sun must be in the right positions to fulfil the conditions of the law of reflection. A much more beautiful and astonishing effect of reflection is to be seen, however, under favourable circumstances under the steep clay escarpment of the hill on which Mr. Fitz-Gerald's house stands. This clay escarpment appears from my window to be of a uniform reddish-yellow colour, with patches of green bushes growing upon it. But when the surface of the water at the base of the cliff is not agitated by wind, and the afternoon sun is shining brightly on the cliff, the reflection in the water is coloured in the most gorgeous

manner with a beautiful intermingling of yellow, orange, red, and green, so intertwined and mingled that the effect is equally beautiful and astonishing. How comes it that the reflection of an object which seems uniform and dull in colour presents such a varied and brilliant appearance? The explanation I take to be as follows: The cliff—although, at the distance of over a mile from which it is viewed, it seems to be of a uniform and somewhat dull colour—is in reality composed of small pieces of clay, earth, and stone of very varied and comparatively pure colours. These are blended by distance into a uniform reddish-yellow; but each spot on the cliff, each coloured piece of clay or rock or green bush, is reflected in the water, and reflected not only once, but a great many times, on the numerous mirrors presented by the surfaces of the small ripples. This multiple reflection of each separate bit of colour enlarges it, and so makes that colour evident to the eye viewing the reflections from a distance, although the actual spot giving off the coloured rays which are thus repeated so frequently on the water-surface is too small to be detected by the eye at that distance. I think that wonderfully close observer of nature, the great painter Turner, has, in one of his pictures of the Venice Lagoon, illustrated in some degree this peculiar effect of reflections from water when the surface is smooth but undulating; but I have never observed the effect so strikingly manifested as in the reflection of that clay cliff below Mr. FitzGerald's house, and I do not think that it has been generally noticed or recorded by landscape-painters. There is to most minds a great pleasure and satisfaction in tracing effects back to their causes, even although we may be able only to take a few steps backwards, to detect a few of the last links of the long chain of causation which has brought about the result. This is all I have attempted to do on the present occasion. To do more would be to go over again, very imperfectly, the ground covered by the distinguished authors of the latest books on light (such as those of Sir George Stokes and Professor Tait), which explain the present state of our knowledge of this subject, whether actual or hypothetical, and also where even hypothetical knowledge ends and ignorance begins. I should have to attempt to answer such questions as these:—1. What is light? 2. What is the cause of the light of the sun? 3. How does this light travel from the sun to the earth? 4. What are the laws of reflection and refraction of light, and what are the causes of these laws? 5. What is the structure of the human eye? 6. How is the impression received by the eye conveyed to me? 7. What is life, which enables me to receive the information conveyed by light to the Qye, and from it by nerves and brain to me?—and many other

such questions, to some of which only can comparatively satisfactory answers be given, in the present state of our knowledge—perhaps to none of them full and certain answers. It was a very wise and happy determination on the part of Lord Salisbury to select as the subject for his most valuable address at the last meeting of the British Association scientific ignorance, rather than scientific knowledge. Not that he might discourage or check research into all the mysteries of nature; rather to stimulate and encourage it by showing what an immense extent of undiscovered country still remains to be explored; while giving at the same time a timely hint to some who seem to imagine that modern science is more perfect, homogeneous, and complete than it really is, notwithstanding the very great progress that has been made in all directions during this nineteenth century. To any of our members who have not read that address and the speeches which followed it I would venture to recommend its perusal. It was printed in the London Times of the 9th August last, and doubtless it has appeared in many other publications.