USE OF THE COMPOUND MICROSCOPE 33 



when we have to deal with opaque substances we 

 usually do away with mirror illumination, and 

 arrange for them to be lighted by other means. 

 Suppose we wish to examine a portion of the wing 

 of a butterfly, so as to observe the shape, colour, 

 and arrangement of its covering scales. The object 

 is placed on a glass slip, and covered with a cover- 

 glass or another glass slip, and put in position on 

 the stage. We attempt to illuminate it by light 

 reflected by the substage mirror, but find that, 

 although possibly a very little light gets through 

 the object, it is not sufficient or satisfactory. To 

 get the best results, we must throw light on to the 

 object from above, and not attempt to pass any 

 through it. In Messrs. Watson's " Naturalist's " 

 microscope, the substage mirror can be removed 

 from its ordinary position and placed in a socket on 

 the stage, so that it may reflect light on to the object. 

 But the commonest method of illuminating opaque 

 objects is by means of a bull's-eye condenser 

 (Fig. 11) — an accessory which is often supplied with 

 a microscopist's outfit, and which can be obtained 

 in various sizes and mountings from 7s. 6d. upwards. 

 A necessary feature of the mounting is that it should 

 allow the lens to be turned in every possible direc- 

 tion. 



The bull's-eye condenser is a plano-convex lens 

 of crown-glass. Its convex surface is nearly hemi- 

 spherical. In using it for illumination of opaque 

 objects, the lamp should be raised as high above 

 the level of the stage as it can be without being so 

 situated that the objective will cause a shadow on 

 the object. The light should be about 10 inches 



5 



