384 



NATURE 



Mar. 19, 1874 



were originally banded ought to become bright and to re- 

 main bright, while those that were originally bright ought 

 to become banded during the rotation of the analyser. 

 The effect to the eye will consequently be a general shift- 

 ing of the bands through one-fourth of the space which 

 separates each pair. Further, as on the one hand plane 

 polarisation is converted into circular right-handed or 

 left-handed by two positions of the plate at right angles 

 to one another ; so on the other right-handed circular 

 polarisation will be converted by the plate in a given posi- 

 tion into plane polarisation having the vibrations in one 

 direction, and left-handed into plane polarisation having 

 the vibrations in a direction at right angles to the former. 

 Hence, if the plate be turned through a right angle from 

 the position first described, the band will be shifted in a 

 direction opposite to that in which they were mo^'cd at 

 first. In this we have evidence not only that the polarisa- 

 tion on either band is circular, but also that on the one 

 side it is right-handed, while on the other it is left- 

 handed. 



All the phenomena hitherto described manifestly 

 depend upon the internal structure of the crystal plate, 

 in virtue of which it affects the vibratory movement of 

 the ether within it differently in different directions. And 

 seeing that most crystals, when broken, divide themselves 

 naturally into smaller crystals having the same form, /.<•. 

 having their planes and edges similarly inclined, we are 

 naturally led to conclude that the structure of these 

 bodies may differ not so much in different parts, 

 as along different lines or planes connected v.ith the 

 forms into which they break, or (as it is also described) 

 with their planes of natural cleav"ge. And this suggests 

 the question whether an uncrystalline body inight not, 

 by pressure, or strain, or other mechanical distortion, be 

 caused to affect the motions of the ether within it in a 

 manner dependent upon their direction, and in that way 

 to (xhibit chromatic effects with polarised light analogous 

 to those described above. E.xperiment answers this 

 question in the affirma'ive. 



The simplest experiment in this branch of inquiry 

 consists in taking a rectangular bar of ordinary glass ; 

 and having crossed the polariser and analyser so as to 

 give a dark field, to strain the bar with both hands as if 

 we were trying to bend it or to break it across. The side 

 towards which it may be supposed to be b-:nt is of 

 course compressed, while the opposite is stretched 

 out. Between these two there must be an intermediate 

 band, more or less midway between the two, which is 

 neither compressed nor stretched. The moment tlie 

 strain is put upon the bar light will be seen to pass 

 through the parts of the bar nearest to both sides, while 

 a band remains dark midway between the two. 



This shows that the mechanical strain has imparted to 

 portions of the glass a structural character analogous, at 

 all events optically, to that of a crystal. The effects may 

 be increased and rendered more striking by placing the 

 glass in a frame furnished with a screw, by which the 

 rod may be firmly held and considerable pressure applied 

 at particular points. When this is done the structural 

 character becomes more completely developed, and the 

 dark band is fringed with colours which appear to flow 

 inwaids or outwards according as the pressuie is increased 

 or diminished. A slightly different, but more effective, 

 e.xhibition of chromatic polarisation is produced by 

 squeezing a thick square plate of glass in a vice. In this 

 case the pressure may be carried further without fear of 

 fracture, and the chromatic effects heightened. 



It is, however, well known that molecular forces, such 

 as those due to heat and cooling, in many cases far 

 transcend in intensity those which we can exert by 

 mechanical arrangements And, in fact, if a block of 

 glass be unequally heated to a very moderate degree, the 

 internal structural effects immediately reveal themselves 

 by dark bands, which indicate the border land between 



strain and pressure. As the block cools, these landmarks 

 gradually disappear, and the field becomes again uni- 

 formly dark. But by far the most splendid effects (and 

 these are permanent) arc produced by unannealed 

 glass ; that is, by glass which has been rapidly and there- 

 fore unequally cooled. When a mass of glass has been 

 cast in a mould in the form of a thick plate, then what- 

 ever be the contour line, the outside will cool first and 

 become a rigid framework to which the interior of the 

 mass must accommodate itself. The nature and direction 

 of the pressure at each point of the interior will be prima- 

 rily dependent upon the form of the contour ; and by 

 adopting various forms of contour the most beautiful and 

 varied figures with coloured compartments may be pro- 

 duced. The forms and colours of the figures produced 

 by transparent bodies when submitted to polarised light 

 have been conversely used as a means of measuring, with 

 almost unparalleled accuracy, the mechanical pressures 

 which such a body is undergoing. 



Besides glass many other substances may be used as 

 reflectors so as to produce polarisation ; for example, 

 leaves of trees, particularly ivy, mahogany furniture, 

 windows, shutters, and often roofs of houses, oil paintings, 

 &c., and last but not least the surface of water. In each 

 of these cases when the reflected beam is examined with 

 a Nicol the alternations of light and darkness are most 

 strongly marked, and the colours (if a crystal plate be 

 used) are most vivid, or in technical language the polarisa- 

 tion is most complete, when the light is reflected at a 

 particular angle. In proportion as the inclination of the 

 incident light deviates from this angle the colours become 

 fainter, until when it deviates, very greatly all trace of 

 polarisation disappears. 



It will befound very interesting to examine the polarisa- 

 tion of sunshine reflected from ripples on the surface of a 

 lake, or better still from the waves of the sea, and its dif- 

 ferent degrees of completeness produced at the variously 

 inclined portions of the w-aves. But without having re- 

 course to nature on so large a scale, an artificial piece of 

 water may be placed in our room. A tea tray will ser\'e 

 as well as anything else to form our little sea ; and a 

 periodic tap at one corner will cause ripple enough for the 

 present purpose. The waves appear bright, and although 

 brighter in some parts than others they are nowhere en- 

 tirely dark. But on turning the Nicol round the contrast 

 of light and darkness becomes much stronger than 

 before. In parts the light is absolutely extinguished, or 

 the polarisation is complete ; in others it is incomplete 

 in various degrees. And if a selenite or other crystal 

 plate be introduced we have the beautiful phenomena of 

 iris-coloured rings playing over the surface of our minia- 

 ture sea. 



Suppose that we now turn our attention to the sky, and 

 on a clear bright day we sweep the heavens with a polari- 

 scope, or even with a mere Nicol's prism, we shall find 

 traces of polarisation in many directions. But if we ob- 

 serve more closely we shall find that the most marked 

 effects are produced in directions at right angles to a line 

 drawn from our eye to the sun, when in fact we are look- 

 ing across the direction of the solar beams. Thus, if the 

 sun were just rising in the east or setting in the west, the 

 line of most vivid effect would lie on a circle traced over 

 the heavens from north to south. If the sun were in the 

 2enith, or immediately overhead, the most vivid effects 

 would be found on the horizon ; while at intermediate 

 hours the circle of strongest polarisation would shift round 

 at the fame rate as the shadow on a sun-dial, so as always 

 to retain its direction at right angles to that of a line join- 

 ing ourselves and the sun. 



Now, what is it that can produce this effect, or indeed, 

 what produces the effect of light from all parts of a clear 

 sky ? The sky is pure space with no contents, save a few 

 miles of atmosphere of the earth, and beyond that the 

 impalpable ether, supposed to pervade all space, and to 



