William Spottiswoode

“ It will, I take it, be our own fault if we spend that leisure in walking through dry places seeking rest; for, to those who have the eyes to see and the spirit to discern, the world is neither dry nor barren ”

“ Suppose that we now turn our attention from the sea to the sky, and that on a clear, bright day we sweep the heavens with our apparatus, or polariscope, as it is called, we shall find traces of polarization colors brought out in a great many directions. But if we observe more closely we shall find that the most marked effects are produced in directions at right angles to that of the sun, when, in fact, we are looking across the direction of the solar beams. ”

“ For if either of the Nicols be turned gradually round (and remember that they are both transparent, colorless blocks of crystal) the light gradually fades until, when it has been turned through a right angle, the light is absolutely extinguished. On turning the Nicol farther the light revives, and afterward again fades, in such a manner that in a complete revolution the light is twice at its brightest, and twice is extinguished. Now, light is due to extremely small and rapid vibrations of a very subtle medium, which is supposed to pervade all space. ”

“ But, apart from this ether, which is certainly inoperative to produce the sky appearance as we see it, a very simple experiment will suffice to show that a diffusion, or, as it has been better called, a scattering of light, is due to the presence of small particles in the air. If a beam from the electric lamp, or from the sun if we had it, be allowed to pass the room, its track becomes visible, as is well known by its reflection from the motes or floating bodies, in fact by the dust in the air. ”

“ And it is a consideration which may well stir the hearts of us outsiders of science, especially on an occasion when we come face to face with some of the greatest philosophers of our time, that any one of us, by practising his eye and riveting his attention, may contribute some natural fact, some fragment of knowledge, to the common stock. ”

“ THERE are many ways in which men have looked at life, the higher kind of life, that ideal which each of us forms in his own mind, to which we each hope that we are always tending. But all these various ideas may for the most part be grouped under two heads: the Ideal of Rest and the Ideal of Work. ”

“ If the line of knobs represent the direction of a ray, their motions will represent the vibrations and waves to which the light is supposed to be due. In ordinary light these vibrations may take place in any directions perpendicular to the ray; and the effect of the crystal of which the Nicol is made, is to restrict these vibrations to a particular direction. In the arrangement now before you the first Nicol causes the vibrations to be altogether horizontal. When the second Nicol is placed similarly to the first, it will obviously have no further effect upon the light ”

“ But, without entering upon the somewhat transcendental question of a future life, these ideas and aspirations have a meaning and reality even in the life which we now live. How do we hope to spend the leisure which old age may some day bring? ”

“ In each of these cases the alternations of light and darkness are most strongly marked, and the colors (if a quartz plate be used) are most vivid, or, in technical language, the polarization is most complete, when the light is reflected from each substance at a particular angle. In proportion as the inclination of the light deviates from this angle the colors become fainter, until, when it deviates very greatly, all trace of polarization at last disappears. ”

“ But, before describing it, there is one peculiar feature of all these experiments which must be noticed namely, that the same results would be produced if we changed the positions of the lamp and the screen. The light which is now polarized by the glass or the water, and examined by the Nicol, might equally well be polarized by the Nicol and examined by the glass or the water. And, therefore, if we find that any contrivance will serve for the one purpose, we may conclude that it will serve equally well for the other. ”

“ In all these instances it is clear, from the experiments themselves, as well as from other experiments which form no part of our present subject, that the modifications which light undergoes are due to the internal structure of the crystals used. And it becomes a question of interest whether it be not possible, by some mechanical process, performed upon a non-cystalline substance, such as glass, so far to imitate a crystalline structure as to reproduce some of the optical results already shown. ”

“ These experiments may be multiplied almost indefinitely, and many a long winter evening might be spent in following polarization into other branches of science upon which it has something to say. For example, on examining a variety of vegetable and animal tissues, slices of wood, fronds of fern, scales of fish, hair, horn, mother-of-pearl, etc., with a suitable polariscope, we should find that they exhibit, internally, definite structural characters, capable of affecting the light, which they transmit in the same general way as do crystals. ”

“ If a plate of crystal, such as selenite, be placed between the two Nicols, and turned round in its own plane, it will be found that in certain positions at right angles to one another no effect is produced. ”

“ A plate of the same crystal worked into a concave form is interesting as showing not only that the colors are dependent upon the thickness, but also that when, with an increasing or diminishing thickness of crystal, they have run through their cycle, they begin again ”