Abril 1 6, 1885] 



NATURE 



567 



accordingly ranged somewhere about the thousandth of an inch, 

 such being the diameter of the spores. The stratum was visibly 

 thicker in those crystals which showed their bright band in the 

 red than in those which showed it in the blue. 



12. That the thin twin stratum was in fact the seat of the 

 colour, admitted of being proved by a very simple experiment. 

 It was sufficient to hold a needle, or the blade of a penknife (I 

 will suppose the latter), close to or touching the surface of the 

 crystal while it was illuminated by light coming approximately 

 in one direction, suppose from a lamp, or from a window a little 

 way off, and to examine the shadows with a watchmaker's lens. 

 The light reflected from the crystal comes partly from the upper 

 surface, partly from the twin stratum, partly from the under 

 surface, which, however, may be too irregular to give a good 

 reflection. The twin stratum is much too thin to allow of 

 separating the light reflected from its two surfaces in an obser- 

 vation like the present, and it must therefore be spoken of as 

 simply a reflecting surface. Corresponding to the three reflecting 

 surfaces are three shadows, where the incident light is cut off: 

 (1) from the upper surface, (2) from the twin stratum, (3) from 

 the under surface. By examining these shadows in different 

 crystals and under varied conditions, it is shown beyond doubt 

 that the coloured reflection comes from the twin stratum. 



The conclusion was confirmed by observations made with sun- 

 light ; but the simple method of shadows is quite as good, and 

 even by itself perfectly satisfactory. 



13. Another useful method of observation, not so very simple as 

 the last, is the following. A slit, suppose horizontal, not very 

 narrow, is placed in front of the flame of a lamp at some 

 distance, and an image of the slit is formed by a suitable lens, 

 such as the compound achromatic objective of an opera-glass. 

 The crystal is placed so as to receive in focus the image of the 

 slit, being inclined at a suitable angle, usually in a plane per- 



ular to the plane of symmetry. The eye is held in a 

 pi .sition to catch the reflected light, and the images formed by 

 the different reflections are viewed through a watchmaker's 

 lens. If the slit be not too broad, the images formed by 

 reflection from the upper surface, from the twin stratum, and 

 from the under surface are seen distinct from each other, so that 

 the light reflected from the twin stratum may be studied apart 

 from that reflected from the upper and under surfaces. 



In this mode of observation it can readily be seen, by turning 



1 ystal in its own plane, and noticing the middle image, 



which is that reflected from the twin stratum, how very small a 



11 out of the position in which the plane of incidence had 

 been the plane of symmetry suffices to re-introduce the coloured 



which had vanished in that critical position, which appears 

 to be a position not merely of absence of colour, but of absence 

 of light altogether ; at least if there be any it is too feeble to lie 

 seen in this mode of observation, though from theoretical con- 

 siderations we should conclude that there must be a very 

 little reflected light, polarised perpendicularly to the plane of 

 incidence. 



14. 1 In allowing a strong solution of chlorate of potash in 

 hot water to crystallise rapidly, in which case excessively thin 

 plates are formed in the bosom of the liquid, I noticed the play 

 of colours by reflection mentioned by Professor Wills as belong- 



the crystals in general at an of their growth. 



This, however, proved to be quite a different and no doubt a 

 much simpler phenomenon. The difference was shown by the 



•Uion of the li^ht, and above all by the character of the 

 spectrum of the light so reflected, which resembled ordinary 

 spectra of interference, and did not present the remarkable 

 character of the spectra of the peculiar crystals. 



15. When, however, the whole was left to itself for a day or 

 so, among the mass of usually colourless crystals r. few were 

 found here and there which showed brilliant colours. These 

 colours were commonly far more brilliant than those of the 

 crystals mentioned in the preceding paragraph, and they showed 

 to perfection the distinctive character of the spectrum of the 

 peculiar crystals. It would have been very troublesome, if 



' possible at all, to examine the twinning of such thin and tender 

 plates as those thus obtained by working on a small scale; but 

 the character of the spectrum, which is perhaps the most 

 remarkable feature of the phenomenon, as well as the depend- 

 ence of the colour on the orientation, may be examined very 

 well ; and thus any one can study these features of the phenom- 

 enon, though he may not have access to such fine coloured 

 crystals as those sent me by Professor Mills. 



10. A certain amount of disturbance during the early stages 



of crystallisation, whether from natural currents of convection 

 or from purposely stirring the solution, somewhat gently so as not 

 to break the crystals, seems favourable to the production of the 

 peculiar crystals. When the salt crystallised slowly from a quiet 

 solution I did not obtain them. 



17. As it is easy in this way, by picking out the peculiar 

 crystals from several crystallisations, to obtain a good number of 

 them, the observer may satisfy himself as to the most usual 

 character of the spectrum. It is best studied at a moderate 

 incidence, as it] is sharper 'than when the incidence is con- 

 siderable. 



iS. The number of coloured crystals obtained by crystallisations 

 on a small scale, though very small, it is true, compared with 

 the number of colourless ones, was still so much larger than 

 Prof. Mills's description of the rarity of the crystals had led me 

 to expect, that I at one time doubted whether the simply 

 twinned crystals which are so very common, if taken at a period 

 of their growth when one component is still very thin, and of 

 suitable thickness, might not possibly show the phenomenon, 

 though the thin twin was in contact on one face only with the 

 brother twin, the other face being in the mother-liquor or in air. 

 The circumstances of reflection and transmission at the first 

 surface of the twin plate must be very different according as it is 

 in contact with the brother crystal, or else with the mother- 

 liquor, or air, or some other fluid ; and yet the peculiar spectrum 

 was shown all the same whether the crystal was in air, or im- 

 mersed in the mother-liquor, or in rock oil. However, to make 

 sure of the matter I took a simply twinned crystal, and ground 

 it at a slight inclination to the C face till the twin plane was 

 partly ground away, thus leaving a very slender twin wedge 

 forming part of the compound crystal, and polished the ground 

 surface. On examining the reflected light with a lens, no colour 

 was seen about the edge of the wedge, where the thickness of 

 the wedge tapered away to nothing; and that, although the 

 bands seen near the edge in polarised light, which was subse- 

 quently analysed, showed that had colours been producible in 

 this way, as they are by a thin twin stratum, they would not have 

 been too narrow to escape observation. 



In another experiment a simply twinned crystal was hollowed 

 out till the twin plane was nearly reached. The hollowing was 

 then continued with the wetted finger, so as to leave a concave 

 smooth surface, the crystal being examined at short intervals in 

 polarised light as the work went on, so as to know when the 

 twin plane was pierced. But though in this case the twin plane 

 formed a secant plane, nearly a tangent plane, to the worked 

 surface, and near the section the twin portion of the crystal must 

 have been very thin for a breadth by no means infinitesimal, as 

 was shown by examination in polarised light, yet no colours 

 were seen by reflection. I conclude therefore that the produc- 

 tion of these colours requires the twin stratum to be in contact 

 on both its faces with the brother crystal. 



19. The fact that a single bright band is what most usually 

 presents itself in the spectrum of the reflected light, though 

 sometimes two or three such bands at regular intervals may be 

 seen, seems to warrant us to regard that as the kind of spectrum 

 belonging to the simplest form of twin stratum, namely, one in 

 which there are just the two twin surfaces near together. The 

 more complicated spectra seem to point to a compound inter- 

 ference, and tu be referable to the existence of more than two 

 twin planes very near together ; and in fact in some of the 

 crystals which showed the more complicated spectra, and which 

 were broken across, I was able to make out under the microscope 

 the existence of a system of more than two twin planes close 

 together. Restricting ourselves to what may be regarded as the 

 normal case, we have then to inquire in what way the existence 

 of two twin planes near together can account for the peculiar 

 character of the spectrum of the reflected or transmitted light. 



Section II. — Of the Proximate Cause of the Phenomenon. — 20. 

 Though I am not at present prepared to give a complete explan- 

 ation of the very curious phenomenon I have described, I have 

 thought it advisable to bring the subject before the Society, that 

 the attention of others may be directed to it. 



That the seat of the coloration is in a thin twin stratum, admits 

 I think of no doubt whatsoever. A single twin plane does not 

 show anything of the kind. 



For the production of the colour the stratum must be neither 

 too thick nor too thin. Twin strata a good deal thicker than 

 those that show colour are common enough ; and among the 

 crystals sent to me I have found some twin strata which were a 

 good deal thinner, in which case the crystal showed no colour. 



