112 PEACTICAL TREATISE ON THE MICROSCOPE. 



light passes is blackened, so that no other rays than those 

 from the mirror should interfere with the illumination. The 

 use of the diaphragm is to modify the rays reflected from the 

 mirror, and to limit the angle of the pencil of light allowed to 

 fall on the object under examination. 



When a very bright light is employed for some time, the 

 eye will often sufier greatly from fatigue, and when taken 

 away from the instrument, a dark spot wiU be seen upon any 

 object that is white ; to remedy this inconvenience, a piece of 

 grey or neutral tint glass may be placed over the hole in the 

 fixed plate, and when the light is passed through either of 

 these, it is so very much softened, that the relief afforded to 

 the eye is truly astonishing. 



Dark Chamber. — This instrument, like the diaphragm, is 

 fitted to the under surface of the stage, and is represented by 

 fig. 57 ; it consists of a plate of brass, c, into which is soldered 

 a short piece of tube, having a dia- 

 phragm or stop, a, in which is an 

 aperture equal in area to the field of 

 view of the lens, and no larger; 

 below this is a sliding tube, b, with 

 an aperture rather larger than that 

 at a ; this last can be moved up and 

 down until the light at a is of the greatest intensity, the aper- 

 ture at a being always in proportion to the size of the lens 

 employed ; this instrument is the contrivance of Mr. Varley, 

 and is described by him in the forty-eighth vol. of the Trans- 

 acUons of the Society of Arts. He applies it always to his 

 instruments, and on account of there being no lens in its con- 

 struction, the light is not decomposed ; he, therefore, prefers 

 it to the Wollaston light for a condenser. It is always em- 

 ployed with his phial-holder, and will be again alluded to. 



Wollaston Condenser, — This instrument, like the preceding, 

 is also fitted to the under surface of the stage ; it consists of a 

 short tube, in which a planoconvex lens, of about three-quar- 

 ters of an inch focal length, is made to slide up and down ; 

 this apparatus is represented in section by fig. 19, or as 

 applied to a microscope in fig. 38, where the lens, set in a 



