DOUBLE PRISM ILLUMINATION. 



177 



P. Rigidum may "be brought out with ease by simply 

 inclining the prism to the angle necessary to cause the 

 shadows of this series of lines to fall directly opposite to 

 the source of light. 



" The method of using this combination will be readily 

 understood by referring to fig. 103. 



"The microscope (a) is placed at the usual angle of 

 inclination ; a valve — say, of N. fihomboides — is selected 

 which lies vertically on the stage ; the front lamp (c) is 

 adjusted so that the centre of the flame shall be eleven 

 inches above the table, and ten inches from the object ; 

 an object glass of moderate angle, such as a Eoss's old 

 J inch, with 60° of aperture, will be found convenient 

 for this first step in the process ; the front prism (d), with 

 its convex surface turned towards the lamp (c), is placed 

 H — 2 inches from the stage, and so adjusted that the 

 pencil of transmitted light shall fall with the utmost 

 obliquity on the object ; now (referring to the former 

 fig. 102), the angle c b a being about 40°, or greater than 



Fig. 103. — Mr. Newton Tomkins' Double Prism Illumination. 



half the angle of aperture of the lens, the valve under 

 examination will be brightly illuminated on a black- 



N 



