TRANSACTIONS OF THE SECTIONS. 



33 



It should first be mentioned that the eyepiece or niicrometer-tube is made double, 

 an outer parallel tube and an inner taper one ; and it is between these two tliat 

 it is required that the light should be brought to the four prisms or micro- 

 meters, any light shining into the inner tube doing mischief by injuring the black- 

 ness of the field. 



On the lens used to give a slight convergence to the light is placed a circu- 

 lar opaque disk o, of a certain size easily ascertained ; a lens of a suitable focus 



Fi<r. 2. 



Line of rays from 

 objectiye. 



From lamp for dark field illumi- 

 nation of micrometer. 



being then placed near the reflector, an image is formed of that opaque disk just 

 over the eyepiece-tube at X, and of such a size, when properly adjusted, that no 

 light can possibly enter the inner tube. 



Thus while not a single ray of light can by any possibility enter the inner tube, 

 a flood of light is sent down between the inner and outer tubes and directed upon 

 the four prisms in whatever angular position they may be. 



It only remains to say that both the intensity and colour of the light for both 

 characters of tlie illumination are under complete control of the observer while 

 actually observing. 



