30 TEXT-BOOK OF BACTERIOLOGY. 



ately become fully invisible; the refractive differences have ceased 

 to exist, the rays of light pass through an optically uniform me- 

 dium, and all loss of light by refraction or reflection is avoided. 



Of far greater importance, however, for the value of these sys- 

 tems is the circumstance that by perfecting the co-efficient of 

 refraction of the intermediate stratum the size of the numerical 

 aperture (that is to say the defining power) is very considerably 

 increased, thus insuring to the oil immersions a decided superiority 

 over all other systems. 



The details of these matters are very difficult ajid very compli- 

 cated. The endeavor has been to point out the most essential facts, 

 but it will be seen how extremely important they are for the proper 

 use of the microscope, -and no one who frequently uses oil immer- 

 sion should neglect to make himself acquainted with the power of 

 his microscope and the principles on which that power depends. 



The oil immersion, in its present form, may be said to have been 

 first contrived by Abbe, and then brought into general use, for bac- 

 teriological purposes, by Koch. This is also the case with the 

 second necessary auxiliary to a " bacteriological microscope " the 

 condenser, or the special illuminating apparatus. 



This too was constructed by Abbe, and has been named after 

 him, but Abbe's apparatus owes its universal application to the 

 recommendation of Koch, who in 1878 regarded it as essential for 

 the examination of micro-organisms. 



The principle of Abbe's apparatus is comparatively easy to un- 

 derstand, the more so as it is easy to illustrate it by a few exam- 

 ples. Take two microscopes, one of which is provided with Abbe's 

 apparatus and the other with only the source of light offered by a 

 movable reflector. Place the same object under both; for instance, 

 an uncolored section of a Guinea-pig which has died of inflammation 

 of the kidney. 



The sections must have been taken out of alcohol, placed in 

 cedar oil, and mounted in Canada balsam. Let a drop of oil fall 

 on the cover-glass, dip the immersion lens into it, and examine the 

 preparation. 



With the microscope unprovided with any special illuminating 

 apparatus, but having the ordinary reflecting mirror, a well-known 

 image to those familiar with histological work will be seen. The 

 nuclei and the cell divisions, the striped intermediate substance 

 arranged in bundles, the layers of the walls belonging to the larger 

 vessels, the network of capillaries, etc., are seen in a word, the struc- 

 ture of the tissue will be apparent. 



This is the "structure picture" of the object, as Koch has called 



