76 



Then the angle d = 90° + the angle c. 



To find the value of x, cosine of angle c : a : : radius : x. 



To find the focus of the lens required, radius : — : : cotangent of 



& 



— 2 — _ : focus + thickness of lens. 

 2 



For the diameter of the lens, / 2s* = diameter. 



A square diaphragm, c g, equal in size to the square surface, d q, 

 should be placed under the prism, as shown in the figure, that is, 

 with one side coinciding with the angle, c, of the prism. 



When in use the prism must be fixed under the stage of the 

 microscope in a similar manner to that adopted with the achromatic 

 condenser, it requiring all the motions of that piece of apparatus, 

 and capability of being revolved on its axis in addition. In my 

 own instrument the prism screws into the fittings of the condenser, 

 in lieu of the lens, and the tube, carrying the rack and pinion, is 

 made to revolve. 



It of course requires centering to the object-glass in use, exactly 

 in the same way as the achromatic condenser, as, if not properly 

 adjusted, the spot of light goes out of the field of view on revolving 

 the prism. This adjustment can, however, be very readily accom- 

 plished, and when once effected the light can be made to fall in any 

 desired direction on the object, without in any way altering the 

 disposition of the lamp or mirror, by simply rotating the prism. 



These prisms can be readily applied to any of the instruments 

 now in use, and the light produced, or rather affected, by them is 

 much stronger than can be obtained from using the mirror to throw 

 it obliquely. The degree of obliquity can also be made greater than 

 by the mirror, owing to the thickness of the compound stage inter- 

 fering with the latter, but not with the former. Of course the same 

 precautions should be observed, when using these prisms, in noting 

 observations to make the requisite allowances for the obliquity of 

 the light, as would have been necessary had the obliquity been 

 obtained in the usual manner. 



By the assistance of one of Nachet's prisms, constructed according 

 to the formulse previously given, I have been able to see with a 

 good quarter-inch objective the markings in Navicula angulata, 

 and many minute lines, dots, and other minutiae in numbers of the 

 fossil Infusoria (so called), the existence of which I had previously 

 no idea of. In fact, for this class of objects I can scarcely praise it 

 too highly. 



