hy transmission through Fluids, 



429 



double refracting eye-piece A B C D, fig. 3, and A, figs. 1, 2 

 and 4. This eye-piece consists of a lens C placed at its focal 

 distance from a small aperture at D, so as to give a well-de- 

 fined image of it when transmitted towards the observer's eye 

 at A ; B is a double refracting achromatic analyser of Iceland 

 spar, or of quartz, which is placed within a tube which slides 

 Fig. 1. Fig. 2. Fig. 4. 



into the tube containing the lens, and also fits into the socket 

 of the vernier, it being sometimes desirable not to use the lens. 

 A Nicol's prism, or a tourmaline similarly fitted, may occasion- 



