262 Lord Rayleigh on the Remarkable Phenomenon 



mere broadening of the band might be due to fewness of 

 alternations ; but this case would be distinguished from the 

 other by the accompanying feebleness of illumination. 



On the whole, the character of the reflected light appears 

 to me to harmonize generally with the periodical theory. 

 One objection, however, should be mentioned. It might be 

 supposed that the total number of twin planes was as likely 

 to be odd as to be even. In the former case the layers of 

 crystal on either side of the thin lamina (which is the seat of 

 the colour) would be of opposite orientations. In many crys- 

 tals the character of the twinning is difficult of observation, 

 but I have not noticed any instance of brilliant coloration 

 answering to this description. So far as it goes this argument 

 is in favour of the simple stratum theory ; but, in view of our 

 ignorance as to how the twin planes originate, it can hardly 

 be considered decisive. 



I have also examined a number of what appeared to be 

 simply twinned crystals, kindly sent me by Mr. Stanford, of 

 the North British Chemical Works. The light reflected from 

 the twin plane is not easily observed on account of its feeble 

 character, at least when, as in the experiments now referred 

 to, the incidence is limited by the requirement that the light 

 must enter the crystal at a face parallel to the twin plane. 

 Using, however, the method described by Prof. Stokes (§ 13), 

 I was enabled to separate the reflexions at the twin plane from 

 those at the external surfaces of the crystal. A narrow slit 

 admitted sunlight into the dark room, and was focused upon 

 the crystal by a good achromatic object-glass*. When the 

 obliquely reflected light was examined with a hand magnifier, 

 a ghost-like image corresponding to the twin plane could 

 usually be detected. As the crystal was rotated in its own 

 plane, this image vanished twice during the revolution. 



It is worthy of notice that there is an evident difference 

 both in the brightness and quality of the reflected light ob- 

 tained from different crystals, even though apparently simply 

 twinned. This suggests that, instead of a single twin plane, 

 there may sometimes be in reality 3, 5, or a higher odd num- 

 ber of such in close juxtaposition. In other specimens, 

 affording similar reflexions, the principal thicknesses on either 

 side of a very thin layer are undoubtedly of the same kind, 

 so that the number of twin planes must be even. Here, 

 again, the reflected light exhibited marked differences, when 

 various crystals were examined. In none of those now 

 referred to could the light reflected from the thin layer be 

 observed without very special arrangements. 



* I did not succeed in my first trials when I employed a common lens. 



