AV7Y? OD UC TOR Y. l^ 



plate, called a diaphragm, in which there are holes of 

 various sizes for the regulation of the required light 

 for transparent illumination ; the observer, however, 

 will often find he can obtain just the quantity and 

 quality of light required without a diaphragm, by in- 

 clining the mirror at various angles, or by shading it 

 occasionally with the hand. A beginner will often 

 find difficulty in getting the focus. Many instruments 

 are provided with two adjustments for altering the 

 focus ; these are the coarse adjustment, which is 

 effected by rack-and-pinion motion ; and the fine ad- 

 justment with very delicate motion. In some micro- 

 scopes the coarse adjustment is obtained by moving 

 the body of the instrument with the hand, as in the 

 figure represented ; but that effected by the rack and 

 pinion is far more pleasant to use. 



For the illumination of opaque objects, the con- 

 densing lens attached to the instrument will be found 

 useful, but, in addition to this, it is very desirable to 

 employ another condensing lens, mounted on a sepa- 

 rate stand, and readily moved in any direction. That 

 known as the bulPs-eye condenser is very convenient 

 and useful ; the lens is a plano-convex one, about 

 three inches in diameter, having a short focus. This 

 lens must be turned with its plane surface to the light 

 or lamp, and its convex side towards the object on 

 the stage of the microscope experience will determine 

 the requisite distance : the rays of light passing through 

 the bull's-eye will form a bright luminous spot upon 

 the object. There are various other contrivances for 

 illuminating opaque objects, but the beginner need 

 not trouble himself with them ; the more simple and 

 the fewer the appliances, the more progress the stu- 

 dent will at first make. I have enumerated, I think, 

 nearly all the apparatus you will find necessary, unless 

 I mention a earner a hicida, or a neutral tint glass 



