i66 



PRACTICAL MICROSCOPY. 



with the D ocular. It must not be forgotten, however, that 

 the use of the erector disturbs the corrections of the objec- 

 tive, producing spherical and chromatic aberrations. This 

 is so great that, when using the A eye-piece and a i-inch 

 objective, an object may be brought to an indifferent focus 

 at any distance between i inch and 2^ inches from the front 

 lens upon the instrument shown in Fig. 24 ; but at \\ inches 

 perhaps the sharpest image is formed. The way to remedy 

 the effect of these aberrations is to use the C or D eye- 

 pieces, when the object runs in and out of focus very 

 sharply, and as the peripheral rays are not gathered in, 

 there is but little aberration of either kind visible. 



The following table will show the use of the erector and 

 draw-tube, used upon the stand in Fig. 24 with a -inch 

 objective of 50 air angle. 



For cheapness, there is nothing better than the watch- 

 maker's or engraver's eye-glasses. These may be fixed 

 into any form of stand the ingenuity of the student may 

 devise, and with these alone very much work can be done, 

 quite as much perhaps as he will be able to execute for 

 some time. 



The subjects for dissection are usually operated upon 

 under water or some other fluid, in dilute alcohol, or even in 

 glycerine. This is a very convenient medium in which to 



