OP THE MICROSCOPE. 



147 



ILLUMINATION OF OPAQUE OBJECTS. 



Diffused Light. This term is applied to ordinary day- 

 light or lamp-light, allowed to fall on the object without the 

 intervention of any special means of concentration. That dif- 

 fused light may be available for the illumination of objects, it 

 is necessary that the objectives be good. Objects which, with 

 ordinary triplets of low angular aperture, are entirely invisible, 

 become beautifully distinct when a better class of objectives is 

 used. Under favorable circumstances the view obtained in this 

 way of any well marked object is very pleasant. 



Bulls-Eye Condenser. This is a large lens of compar- 

 atively short focus, which is made to condense the light on the 

 object in the same way that the common burning-glass acts, but 

 with effects greatly less marked, since the light is so much less 

 intense. In some cases the condensing lens is attached to the 

 microscope, and in some special cases this is very convenient, 

 but where there is only one condenser, it should be mounted on 

 a stand, as shown in Fig. 43, so that it may be 

 placed at any height and turned in any direc- 

 tion. Placed between the object and the 

 lamp, it collects the rays of the latter to a 

 focus which brightly illuminates any object 

 upon which it may fall. Opaque objects, 

 which by diffused light are barely visible 

 under the microscope, become very distinct 

 and clearly defined when thus illuminated, 

 and many of them, such as the wings of in- 

 sects and certain minerals, appear in the most 

 gorgeous colors, which, however, are perfectly 

 natural, and are not the result of chromatic 

 defects in the lenses. 



In viewing an opaque object by reflected 

 light, it is evident that we are enabled to 

 judge of the irregularities of the surface 

 largely by means of the shadows cast by the 

 prominences. By raising or lowering the lamp, and also the con- 



Fig. 43. BULLS-EYE 



