TEXT-BOOK OF BACTERIOLOGY. 31 



it. This picture is seen because parts of the tissue — the nuclei, the 

 fibres, the capsules of the glomeruli, etc. — differ from the Canada 

 balsam in refractive power. Thus they cause by the diffraction of 

 the light passing through them a picture consisting of lines and 

 shadows — a structure picture. If their optical qualities were the 

 same as those of the media surrounding them, it would not be pos- 

 sible to see them at all. 



That such is really the case may be demonstrated by looking 

 through a microscope provided with an "Abbe." Then nothing is 

 left of the tissue — only an even light yellow, translucent layer is 

 seen, in which no particular features can be distinguished. 



That is the effect produced by Abbe's apparatus : it effaces the 

 structure picture; it removes the differences of refraction on which 

 it depended by throwing a strong, widely-opened cone of rays upon 

 the centre of the field of vision. Abbe's condenser is a compound 

 illuminating lens, which gives a cone of rays extremely broad in 

 comparison to its length, and with a very wide angle of aperture for 

 the rays which issue from it. This causes the disappearance, in the 

 illuminated picture, of all that depended on diffraction. 



What advantages this gives in examining bacterial prepara- 

 tions is obvious. If the unstaiiied sections be removed and replaced 

 by two others which have been previously' stained with gentian- 

 violet and the instrument used without a condenser, the structure 

 picture is seen in an unaltered form. The details of the tissue are 

 again seen in their peculiar plastic forms. Yet certain parts of the 

 ob.iect now stand out with peculiar distinctness, particularly those 

 which are saturated with the coloring matter. Thus the cell nuclei 

 in particular are detached from their surroundings, and with them 

 and abundantly distributed throughout the whole preparation are 

 certain uniform colored rods, the stained anthrax bacilli. 



In fact, we have two pictures of different origin, and also of dif- 

 ferent value, competing with each other — the structure picture and 

 the so-called "color picture; " the latter, quite independently of the 

 former, displaying certain parts distinguished by a special suscep- 

 tibility for receiving color. While one is caused by difference of 

 refraction, the other is produced by processes of absorption. That, 

 however, the structure picture and the color picture antagonize 

 each other, so to speak, may be observed without difficulty if a 

 glance be taken once more through the microscope provided with 

 Abbe's apparatus. The condenser wipes out the structure picture, 

 and now only the pure color picture is observed. The bacteria in 

 the latter are far cleaner and more distinct; their number, too, 

 seems to have increased, their fine distinctions of form have become 



