90 MR. J. SMITH ON THE ORIGIN OF COLOUR 



sections of crystals^ when light is allowed to fall upon 

 them in one direction only. 



139. There is another variety of this experiment. Make 

 a similar figure^ but in place of cutting out any of the 

 segments, cut the arc of one completely through, but the 

 chord or diameter only half through the thickness of the 

 board, so as to enable it to be bent backwards. Bend it so 

 as to be at right angles to the other segment. The effect of 

 this is very beautiful, and the generated figure extremely 

 elegant. It may be mentioned, by the by, that all the gene- 

 ated figures are beautiful, and may be called gems of art. 



140. From a great variety of coloured diagrams three 

 have been selected to be lithographed as specimens of the 

 figures as well as of the colours produced by these experi- 

 ments. Plate T. is a representation of the effect produced 

 hj fig. 10a and 10b, plate V. In a paper such as this where 

 the subject is only briefly, and for the first time presented 

 to the public, I could have wished that some more simple 

 figure, more like a cut crystal, had been selected. But 

 as it is, let us consider A and B as the obverse and 

 reverse sides of a card, and that they represent, when in 

 motion, a transparent solid body, being cut so that when 

 held with the obverse to the sight the dark part appears 

 as a plane or planes representing no-light, and the white 

 part as a plane reflecting light ; and, vice versa, when the 

 reverse is held to the sight the part which was white be- 

 fore appears now to be black or dark, and what was dark 

 before to be white. Centre the card exactly on the point 0, 

 so that when in motion it may be perfectly perpendicular, 

 the obverse repeating itself precisely on the reverse and 

 the reverse on the obverse, and we shall obtain a figure 

 coloured as in plate I. 



The principle on which the colours are produced is as 

 easily understood from this as from a figure of more crys- 

 talline form. The different colours are obviously produced 



