136 MICROSCOPIC ILLUSTRATIONS. 



ment, in order that a fine definition may be obtained with 

 them. "The centre of the aperture must be in the 

 common axis of the lenses" of the microscope, " and the 

 image of the perforation formed by the condensing 

 lens must be brought by such adjustment into the 

 same plane as the object to be examined*." 



Stops or diaphragms serve to increase the definition of 

 the outline of delicate objects. Thus, the cilia and inter- 

 nal structure of animalcules become much more visible 

 by their aid than without it, especially when we have 

 single magnifiers only. They enable us, also, to reduce 

 the quantity of light, when using achromatic microscopes 

 with object-glasses of long foci ; by which reduction our 

 observations on thin transverse sections of wood, as well 

 as on cylindrical and spherical bodies, may be more 

 accurately made. 



With high magnifying powers it is often desirable to 

 have a lateral adjustment for the Wollaston illumination, 

 to effect which, a movement to and fro must be given to 

 the screw which holds the socket, on the under side of 

 the moveable stage (page 105) ; whilst the body admits 

 of a lateral adjustment, to the right or left in the other 

 direction. By these means we may obtain either a cen- 

 tral or eccentric pencil of light. 



The reader must bear in mind, that stops have the 

 effect of decreasing the angle of aperture of the object- 

 glass ; arid therefore, in testing it, he must make proper 

 allowance for them. 



* Philosophical Transactions, p. 9, 1830. 



