DESCRIPTION OP THK ACHROMATIC MICROSCOPE. 119 



Talbot's " Analytic Crystals." As these experiments, 

 however, are full of interest, I have judged it advisable 

 to extract from the Philosophical Transactions, and give 

 entire, in the Appendix, Mr. Talbot's own papers upon 

 the subject. 



If, in place of the upper prism, or analyzer, a double- 

 image prism be fixed at P, another and very beautiful 

 series of effects will result from the exchange. With this 

 modification there will at the same instant be seen two 

 images of the object arrayed in their complementary 

 colours ; and if we turn this prism round in the manner 

 before described, an interchange of colours will take 

 place between the images, the one assuming that of the 

 other. If long needle-shaped crystals are procured for 

 the purpose, the two images will be so far separated as 

 to appear quite distinct. They will vary in their positions 

 also, as well as in their colours and intensity, while the 

 prism is being turned. A moon-shaped or crescent form 

 of complementary colours w T ill be discernible on each 

 side of the field of view, while the latter will be white, 

 from the union of the two complementary colours. The 

 most gorgeous effect of all, however, is produced by this 

 double-image prism when a plate of quartz is inserted 

 below the crystals (say crystals of chlorate of potass) 

 placed at a, b. In short, the whole experiment is so 

 beautiful, that it is in vain to attempt any thing like a 

 description of it. 



Some crystals possess the peculiar property of exhi- 

 biting two colours, when viewed by polarized light; 



