THE MICROSCOPE 39 



under Objectives. The simplest method of marking 

 oculars is by indicating the number of diameters (or 

 times) they magnify the image formed by the object- 

 ive. In this system the powers range from 5X to 20 X . 

 Where high magnification with long working distance 

 is desired, high-power oculars may often be used to 

 advantage. The better types of oculars are corrected 

 for spherical and chromatic aberration. - The mag- 

 nifications obtainable with different combinations of 

 objectives and oculars are stated in tabular form in 

 the Appendix. 



Condensing Lenses and Mirror. — The purpose of 

 the mirror is to reflect light upon the object, thus 

 illuminating it upon all sides. Illumination can be 

 secured without use of the mirror; but upon viewing 

 the object it will be found that one side is light and the 

 other is dark. That portion of the specimen nearest 

 the source of light is clearly defined, while that further 

 away is obscured. Furthermore, in most instances 

 we desire views of cell structure which can only be 

 obtained by light passing through the object. One 

 surface of the mirror is plane and the other is concave. 

 The plane surface reflects parallel rays and the con- 

 cave surface reflects converging rays, or concentrates 

 the light. The plane surface is usually used where 

 the source of light is near, and with instruments 

 equipped with condensing lenses. The concave sur- 

 face is used when the source of light is distant. It 

 is also used when interfering objects, such as window 

 frames or nearby buildings, partially cut off the light 

 or are projected into view. When possible, the plane 

 surface should be used when working with instruments 

 equipped with condensing lenses. 



