L68 THE MICROSCOPE. Nov. 



The Dissecting Microscope. 



By prof. F. M. GOODMAN, Ph.G. 

 To every student of botany a dissecting microscope is 

 indispensable. There are several forms to be found in 

 shops and this leads to frec^uent inquiry concerning the 

 "best." When one is considering which is the best he is 

 led to the consideration of the lenses, the construction of 

 the hand rest, the method of supporting the lens and the 

 means of altering its focus. The lenses usually employed 

 in simple microscopes are either plano-convex or double- 

 convex and lenses of these forms are subject to two in- 

 herent disorders ; they distort the image and decompose 

 light. The first is called spherical aberration, being oc- 

 casioned by the sphericity of the lens. The second is 

 chromatic aberration because the image seen by its aid 

 shows a border of color not present in the object. 



Spherical aberration may be overcome by making the 

 lens much rounder on one side than on the other. This 

 has the effect of rendering the rays of light from an ob- 

 ject parallel and overcomes aberration of form. Using 

 three lenses of the ordinary form tends to correct it, but 

 the usual, because it is the cheapest, way is to place a 

 diaphragm betAveen the lenses, shutting out the peripheral 

 rays and using only the central ones which are normally 

 parallel. Chromatic aberration is overcome by cement- 

 ing lenses of the proper form, of two kinds of glass to- 

 gether. 



One combination of lenses which may be purchased of 

 optical instrument makers is called the achromatic triplet. 

 This is composed of three lenses of two kinds of glass, 

 crown and flint, so adjusted in form and quantity as to' 

 overcome both aljerrations. This gives exquisite defini- 

 tion and is justly regarded an excellent lens for a dis- 

 secting microscope, but it is quite expensive and the 

 magnifying power is not increased over an ordinary 



