MICROSCOPES. 335 



called " bull's eye " condensing lenses or by curved polished silver 

 reflectors. These latter have the advantage of giving the shadows 

 apparently on the true side, and when made of parabolic form ana 

 so fixed that their focus coincides with that of the object glass, 

 the illumination is very satisfactory for many objects. Lieber- 

 kuhns differ from such mirrors in reflecting on the object, not 

 the light which passes over it from one side at right angles to the 

 axis of the instrument, but that which passes on each side in the 

 line of the axis. They cannot therefore be used with very large 

 objects, and it is generally necessary to have under the object a 

 dark stop, to prevent any direct light from entering the instru- 

 ment. Moreover, they throw the light from all sides, so as not to 

 give rise to such well-marked shadows as is desirable. None of 

 these reflectors can be used with very high powers. For them 

 illuminators have been made consisting of a flat plate of thin glass 

 placed above the object-glass, reflecting light down through the 

 lenses, which condense it on the object, and again transmit it to 

 the eye through the reflecting plate of glass. Want of sufficient 

 light is, however, unfortunately the great drawback in the use 

 of very high powers with reflected light, which otherwise would 

 have many advantages, and might enable us to see objects and 

 structures not otherwise distinctly visible. 



The possibility of seeing objects by transmitted light depends 

 on their absorbing or bending it in such a way as to give rise to 

 more or less well-defined outlines or markings. One half of 

 the capabilities of the microscope may easily be lost by not 

 illuminating each particular class of objects in a suitable manner. 

 For comparatively low powers, a flat or concave mirror may 

 be used to reflect the light, but for higher powers it is im- 

 portant that the light should be concentrated by means of an 

 achromatic condenser, so adjusted that the rays of light passing on 

 all sides of the object examined may be under the same conditions 

 as those which come from the object itself. For the separation 

 of very close lines with high powers, it is important that the 



