ADJUNCTS TO THE MICROSCOPE. 



45 



which turns upon its axis, and slides up and down a stout wire 

 affixed to a steady foot. B, is the smaller lens, whose handle 



Fig. 15. 



slides through a socket, working on a hinge-joint. Sometimes 

 a lens of this kind is affixed to the stage of the microscope, 

 when it can be used in combination with the bull's-eye lens, 

 or alone. C, is the object upon which the light is concen- 

 trated. D, the lamp. To condense the light on the mirror, 

 the lens, A, alone is used. The lamp is of the kind called a 

 fountain lamp, and slides up and down a stem, on which it can 

 be fixed at any height by a screw. A shade should always 

 be used with the lamp, in order to screen the eye as much as 

 possible from any light save that which proceeds through the 

 instrument. As it is a matter of much consequence to our 

 observations that we should have a steady intense light, it is 

 not immaterial what kind of oil, &c., we employ. After many 

 trials and disappointments, I am convinced that pure sperm oil 

 is the pleasantest, cheapest, and best. Caraphene, burning- 

 fluid, and gas give out a very intense light, but there is a 

 tremulous motion in the flame, which is somewhat unpleasant. 



