366 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



away. The flat surface thus produced is exactly parallel to the plane 

 surface of the lens. The remainder of the curved surface is silvered. 

 The condenser is brought into optical contact with the object slip by 

 means of a drop of cedar oil. 



The path of the rays of light is shown in fig. 57. The rays (a) from 

 the source of light are reflected by the mirror to {b), and thence to b' 

 and b". The rays c are likewise reflected to b, b', b". 



The stop Bl. cuts off all the illuminating rays of less than 1 • 05 N.A. 

 It is placed close to the under surface of the lens to prevent any disturb- 

 ing reflections. This stop can be turned aside if desired and ordinary 



r -Obj.T 



Fig. 58. 



mirror illumination obtained. From fig. 57 it will be seen that all rays 

 which enter the condenser of JST.A. 1-05 to 1 • 30 are totally reflected by 

 the upper surface of the cover-glass, so that it is quite impossible for these 

 to enter the object-glass directly. The objective can only receive rays 

 which, after reflection at b", have impinged upon the particles of the 

 object and have been diffracted by these from their original direction. 

 These diffracted rays form the. image in the ]\Iicroscope. The reflecting 

 surface of the condenser throws a well lighted image of the source of 

 light in the plane of the object. The object must always be the same 

 distance from the upper plane surface of the condenser owing to the 

 short focus of the latter. This is easily managed by using glass slips of 



