5 66 



NA TURE 



[April 1 6, 1885 



good cleavages parallel to the two P planes and to the C 

 plane. The crystals are very commonly twinned, the twin 

 plane being C. 



2. If one of the brilliantly coloured crystals be examined by 

 reflection, and turned around in its own plane, without altering 

 the angle of incidence, the colour disappears twice in a com- 

 plete revolution. The vanishing positions are those in which 

 the plane of incidence is the plane of symmetry. The colour is 

 perhaps most vivid in a perpendicular plane ; but for a very 

 considerable change of azimuth from the perpendicular plane 

 there is little variation in the intensity of the colour. There is 

 no perceptible change of tint, but on approaching the plane of 

 symmetry the colour gets more and more drowned in the white 

 light reflected from the surface. 



3. If instead of altering the azimuth of the plane of incidence 

 a plane be chosen which gives vivid colour, and the angle of 

 incidence be altered, the colour changes very materially. If we 

 begin with a small angle the colour begins to appear while the 

 angle of incidence is still quite moderate. What the initial 

 colour is, varies from one crystal to another. As we increase 

 the angle of incidence the colour becomes vivid, at the same time 

 changing, and as we continue to increase the angle the change of 

 colour goes on. The change is always in the order of increasing 

 refrangibility ; for example, from red through green to blue. 

 Not unfrequently, however, the initial tint may be green or blue, 

 and on approaching a grazing incidence we may get red or even 

 yellow mixed with the blue, as if a second order of colours were 

 commencing. 



4. The colours are not in any way due to absorption ; the 

 transmitted light is strictly complementary to the reflected, and 

 whatever is missing in the reflected is found in the transmitted. 

 As in the case of Newton's rings, the reflected tints are much 

 more vivid than the transmitted, though, as will presently appear, 

 for a very different reason. 



5. As Dr. Herepath remarked to me long ago, the coloured 

 light is not polarised. It is produced indifferently whether the 

 incident light be common light or light polarised in any plane, 

 and is seen whether the reflected light be viewed directly or 

 through a Nicol's prism turned in any way. The only difference 

 appears to be that if the incident light be polarised, or the 

 reflected light analysed, so as to furnish or retain light polarised 

 perpendicularly to the plane of incidence, the white light 

 reflected from the surface, which to a certain extent masks the 

 coloured light, is more or less got rid of. 



6. The character of the spectrum of the reflected light is 

 most remarkable, and was wholly unexpected. A direct-vision 

 hand spectroscope was used in the observations, and the crystal 

 was generally examined in a direction roughly perpendicular to 

 the plane of symmetry ; but it is shown well through a wide 

 range of azimuth of the plane of incidence. No two crystals, 

 we may say, are alike as to the spectrum which they show, but 

 there are certain features common to all. The remarkable 

 feature is that there is a pretty narrow band, en- it may be a 

 limited portion of the spectrum, but still in general of no great 

 extent, where the light suffers total or all but total reflection. 

 As the ang'e of incidence is increased, these bands move rapidly 

 in the direction of increasing refrangibility, at the same time 

 increasing in width. The character of the spectrum gradually 

 changes as the angle of incidence is increased ; for example, a 

 a single band may divide into two or three bands. 



The bands are most sharply defined at a moderate angle of 

 incidence. When the angle of incidence is considerably in- 

 creased, the bands usually get somewhat vague, at least towards 

 the edges. 



7. The commonest kind of spectrum, especially in crystals 

 prepared on a small scale, which will be mentioned presently, is 

 one showing only a single bright band ; and I will describe at 

 greater length the phenomena presented in this case. 



When the angle of incidence is very small, the light reflected 

 from the reflecting surfaces of the crystal shows only a continuous 

 spectrum. As the angle of incidence is increased, while it is 

 still quite moderate a very narrow bright band shows itself in 

 some part of the spectrum. The particular part varies from one 

 crystal to another ; it may lie anywhere from the extreme red to 

 the extreme violet. It stands out by its greatly superior bright- 

 ness on the general ground of the continuous spectrum, and 

 when it is fully formed the reflection over the greater part of it 

 appears to be total. The appearance recalls that of a bright 

 band such as the green band seen when a calcium salt, or the 

 orange band seen when a strontium salt, is put into a Bunsen 



flame. The bright band is frequently accompanied right and 

 left by maxima and minima of illumination, forming bands ol 

 altogether subordinate importance as regards their illumination. 

 Sometimes these seem to be absent, and I cannot say whether 

 they are an essential feature of the phenomenon, which some- 

 times fail to be seen because the structure on which the bands 

 depend is not quite regularly formed, or whether, on the othei 

 hand, they are something depending on a different cause. 



Disregarding these altogether subordinate bands, and taking 

 account of the mean illumination, it seems as if the brightness 

 of the spectrum for a little way right and left of the bright land 

 were somewhat less than that at a greater distance. 



When the main band occurs at either of the faint ends of the 

 spectrum, it is visible, by its superior brightness, in a region 

 which, as regards the continuous spectrum, is too faint to be 

 seen, and thus it appears separated from the continuous spectrum 

 by a dark interval. 



When the angle of incidence is increased, the band moves in 

 the direction of increasing refrangibility, and at the sain 

 increases rapidly in breadth. The increase of breadth is far too 

 rapid to be accounted for merely as the result of a different law 

 of separation of the colours, which in a diffraction spectrum 

 would be separated approximately according to the .squared 

 reciprocal of the wave-length, while in bands depending on 

 direct interference the phase of illumination would change 

 according to the wave-length. 



S. The transmitted light being complementary to the incident, 

 we have a dark band in the transmitted answering to the bright 

 band in the reflected. In those crystals in which the band is 

 best formed, it appears as a narrow black band even in bright 

 light. When the band first appears as we recede from a 

 normal incidence it is extremely narrow, but it rapidly incn 

 in breadth as the angle of incidence is increased. 



9. Some of the general features of the phenomenon were 

 prettily shown in the following experiment : — 



Choosing a crystal in which the bright band in the reflected 

 light began to appear, as the incidence was increased, on the 

 red side of the line D, so that on continuing to increase the 

 incidence it passed through the place of the line D before it 

 had become of any great width, I viewed through the crystal 

 sheet of white paper illuminated by a soda flame. A dark ring 

 was seen on the paper, which was circular, or nearly so, and was 

 interrupted in two places at opposite extremities of a diameter, 

 namely, the places where the ring was cut by the [Jane of 

 symmetry. The light of the refrangibility of D was so nearly 

 excluded from the greater part of the ring that it appeared 

 nearly black, though slightly bluish, as it was illuminated by the 

 feeble radiation from the flame belonging to refrangibilities other 

 than those of the immediate neighbourhood of D. The ends 

 of the two halves of the ting became feeble as they approached 

 the plane of symmetry. A subordinate comparatively faint ring 

 lay in this crystal immediately outside the main one. 



10. Suspecting that the production of colour was in some 

 way connected with twinning, I examined the cleft edge of some 

 . >f the crystals which happened to have been broken across, and 

 found that the bright reflection given by the exposed surface was 

 interrupted by a line, much finer than a hair, running parallel 

 to the C faces, which could be easily seen with a watchmaker's 

 lens, if not with the naked eye. This line was dark on the 

 illuminated bright surface exposed by cleavage, a surface which 

 I suppose illuminated by a source of light not too large, such as 

 a lamp, or a window at some distance. The plane of incidence 

 being supposed normal to the intersection of the cleavage plane 

 by the C faces, on turning the crystal in a proper direction around 

 a normal to the plane of incidence, the light ceased to be re- 

 flected from the cleavage surface, and after taming through a 

 certain angle, the narrow line which previously had been dark 

 was seen to glisten, indicating the existence of a reflecting 

 surface, though it was much too narrow to get a reflected image 

 from off it. The direction of rotation required to make the 

 fine line glisten was what it ought to be on the supposition that 

 the fine line was the cleavage face of an extremely narrow twin 

 stratum. 



11. On examining the fine line under the microscope, it was 

 found to be of different thicknesses in different crystals, though 

 in those crystals which showed colour it did not vary very greatly. 

 On putting a little lycopodium on the cleavage face interrupted 

 by the fine line, it was seen that in those crystals which showed 

 colour the breadth of the twin stratum varied from a little greater 

 to a little less than the breadth of a spore. The tin,: 



