180 Prof. G. G. Stokes. On a remarkable [Feb. 26, 



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 observation 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. Where the 

 body casting the shadow is pretty broad in one part, as the blade of a 

 penknife, the shadows in part overlap. The shadows are arranged as 

 in the figure, where the numerals mark the streams of light reflected 

 from the portions of the field on which they are respectively written, 

 1 denoting the stream reflected from the upper surface, 2, that 

 reflected from the twin stratum, 3, that reflected from the under 

 surface. 



Let the crystal show by reflection, at the incidence at which the 

 observation is made, say a green colour. Then this green colour is 

 seen in the full field 123, though mixed with the white light reflected 

 from the upper surface. The green is a good deal more vivid in the 

 field 23, as the reflection from the upper surface is got rid of. The 

 green is wholly absent from the fields 3, 0, 13, and 1. The field 3, 

 and perhaps also the field 13, may show a little of the complementary 

 red from transmitted light. The distinction between the fields 12 and 

 123 is not conspicuous, and often cannot be made out. The distinc- 

 tion, so far as it depends on the third shadow, is strongest between 3 

 and 0, and next to that between 13 and 1, We are not obliged, 

 however, to have recourse to the third shadow, which is often difficult 

 to see ; the first two are amply sufficient. 



Suppose we take a crystal which is broken at the edge so as to 

 expose a cleavage surface interrupted by the cleavage of the narrow 



