THE OPTICAL EQUIPMENT 63 



mination appears as a totally reflecting surface. Even a metallic 

 surface is, however, not perfectly opaque, so that structure 

 becomes visible as the result in part of refraction or absorption. 

 The absorption may be general, giving an effect of light and 

 shade ; or it may be selective, with the result that if light of 

 a particular wave-length is absorbed, either over the whole 

 surface or over small portions of it, the image is a coloured 

 one. A complete comprehension of the variations in appearance 

 to be obtained by these methods of above-stage illumination 

 is most essential in order to interpret properly the microscopic 

 appearances of opaque objects. 



Where an object is a perfectly plain and highly polished 

 metallic surface, both methods the oblique and the vertical 

 should be used for its examination. 



With the oblique method the angle of incidence is, to a 

 large extent, determined by the focal length or working dis- 

 tance of the objective ; in fact, the applicability of this method 

 depends entirely on the distance between the front lens of the 

 objective and the object. If this distance is very small, the 

 incident ray becomes so nearly horizontal that little or no re- 

 flection of the light into the objective takes place. For high 

 powers, therefore, the vertical method has to be resorted to, 

 and is practically the only one available. When using objectives 

 of low power certainly not higher than half an inch the 

 method of projecting the image of the radiant directly on to the 

 object by the bull's-eye condenser is practicable. For opaque 

 objects, such as foraminifera, or similar ones, where w T ell-defmed 

 shadows can be obtained, this method works fairly well ; but 

 it is always open to the objection that on one side a hard 

 shadow is obtained from which there is little or no light 

 reflected into the objective. Where no other arrangement 

 than the bull's-eye is applicable, this can only be obviated in 

 part by ensuring that the light is as high above the level of the 

 stage of the microscope as practicable, that is, as high as the 

 object-glass will permit, so that the shadows are reduced to a 

 minimum. The bull's-eye should be in such a position that 

 the smallest possible image of the radiant that will illuminate 

 the field is projected on to the object. 



A simple arrangement that gives good results with opaque 

 objects, and is an improvement on the bull's-eye method alone, 



