8 THE FROG 



and the object. It should be supported in such a way as to 

 leave both hands of the operator free for using the dissect- 

 ing instruments. It may be used for examining an object 

 by either transmitted or reflected light, as explained below. 

 The student should learn the relative advantages of the two 

 methods of illumination as applied to various objects, and 

 use either or both methods as occasion requires. In looking 

 through the lens, keep the eye close to it. 



1. The Use of Transmitted Light. If the object is 

 translucent, or it is desired to bring out sharply the outlines 

 of an opaque object, the dissecting lens and stand should be 

 adjusted so chat the light reflected by the mirror is trans- 

 mitted thiough the glass plate on which the object is 

 mounted, to the eye of the observer. The object is usually 

 mounted on a microscopical object glass which is placed on 

 the glass stage of the dissecting stand. By adjusting the 

 mirror so as to reflect either the light received direct from 

 the window, or the diffuse light of the room, various de- 

 grees of illumination may be obtained. Examine in this way 

 a translucent object such as the wing of a honey-bee, also 

 an opaque object such as the leg of a honey-bee. Both ob- 

 jects should be mounted dry. Is the image formed by the 

 dissecting lens inverted? Are right and left reversed? 



2. The Use of Reflected Light. In case the object i? 

 opaque, and it is desired to examine the general surface as 

 well as the outline, a metallic plate is placed on the glass 

 stage to cut off the light coming from the mirror, and the 

 object is placed on this plate. One side of the plate should 

 be white, the other black; use the side which will afford a 

 background contrasting with the general color effect of the 

 object. Examine in this way a dark object such as the leg 



