86 PRACTICAL MICROSCOPY. 



2 inches in length, I J deep at the front, and 2 inches at the 

 back ; the base plate and ends being made from glass T % of an 

 inch in thickness and cemented together with marine-glue. 

 For higher powers, the form of trough shown in Fig. 74 

 is desirable. It consists of a glass slide, 3 x ij inches, 

 upon which is cemented with marine glue an ebonite or 

 glass semi-rectangular piece as shown in the figure ; the 



half of a flat indiarubber 

 ring will answer admirably, 

 the front being formed of a 

 piece of thin glass. 



In use, nearly fill the trough 

 with water, place it on the 

 FlG stage of the microscope, and 



incline the body of the instru- 

 ment so that the observations may be made with comfort ; 

 adjust the lamp and concave mirror so that the most intense 

 and central light is thrown through the instrument; and 

 lastly, adjust the diaphragm until most of the marginal rays 

 of light are cut off, there remaining only just sufficient light 

 to work with. In all cases apply a larger aperture only 

 when absolutely necessary. 



It may be here stated that the moderate use of the micro- 

 scope, either monocular or binocular, when employed in the 

 above manner, will not injure the eyesight of a healthy 

 person. When, however, an excessive glare of light is con- 

 stantly employed, the eye becomes less sensitive to ordinary 

 light; excess of illumination is a common fault with 

 beginners. 



The several forms of zoophyte troughs such as those 

 shown in Figs. 73 and 74 may be obtained from any maker 

 or dealer in microscopic apparatus. 



Fig. 73 is a wide trough, but it may be narrowed by 

 means of the wedge and spring, which drives a thin glass 



