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III. 0?i ylnalytk Crystals, Bij H. F. Talbot, Esq* 

 [Illustrated by Plates I. II, & III.] 



IN the course of experiments which I made with my po- 

 larizing microscope, I discovered a class of crystals which 

 produce phtenomena of great beauty, but of a nature very 

 different from any that have been before described ; for which 

 reason a new name is necessary for them, and I have pro- 

 posed that of Analytic Crystals. 



I have given a brief description of them in the Philosophi- 

 cal Transactions for 1837 (page 32)f. But as I am well aware 

 of the truth of the Horatian maxim, 



" Segniiis imtant aninios dcmissa per aures 

 Qumn qiuB sunt oculis subjectajidelibus " 



I have here placed before the eye of the reader some coloured 

 figures in illustration of the subject. 



The usual method of displaying coloiu's in crystalline 

 bodies, consists in placing them between two tourmalines, 

 called the j)olarizer and the analyser. 



Instead of tourmalines the reflexion from plates of glass 

 may be used ; but whatever form of instrument is employed, 

 it is necessary to have tnso of them. Each of these must be 

 capable of polarizing common light. Knot, it will not serve 

 the intended purpose. A plate of sulphate of lime, for in- 

 stance, placed vertically to the eye, will not answer either as 

 a polarizer or an analyser. 



The colours displayed by crystals thus placed between two 

 tourmalines are well known to be very complex and numerous. 

 The greatest variety of tints is often seen in the field of view 

 at once. 



Now, the crystals to which my present paper refers differ 

 from the ordinary ones chiefly in two remarkable particulars. 



(1.) They display, in the arrangement which I shall after- 

 wards describe, two colours only at any given time. These 

 colours are complementary to each other, and consequently 

 in the strongest possible contrast. 



(2.) The polarizing plate is employed alone, or in conjunc- 

 tion with a plate of sulphate of lime. And no analysing plate 

 is required. 



From which it will readily appear how differently charac- 

 terized these pha3nomena are from the ordinary appearances 

 of polarized light, although equally beautiful as micro- 

 scopic objects. 



These results may be obtained with great facility in the fol- 

 lowing manner: 



• Communicated l)y the Author. 



[t See Loncl. and Edinb. Phil, Mag., vol. i.s. p. 288 ; vol. x. p. 218.] 

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