TEXT-BOOK OF BACTERIOLOGY. 43 



The ordinary photographic plates are only sensitive to rays be- 

 longing to the blue part of the spectrum; all the others are more 

 or less ignored by the plate. Now, these blue rays are absorbed by 

 a solution of Bismarck-brown. If a preparation treated with vesu- 

 vin be placed before a plate and illuminated, the portions of the 

 object which are stained brown, especially therefore the bacteria 

 and the cell nuclei, will stop the really efficacious rays. At these 

 points the plate receives no light, and the bacteria are seen on the 

 negative as transparent points or lines, according to their shapes. 



These phenomena were, however, only to be seen in the exact 

 manner here described, with the old collodion plates; the dry 

 plates now universally employed possess a higher degree of sensi- 

 tiveness to color, and go beyond the blue portion of the spectrum. 

 Recently plates have been made which possess this quality in a far 

 higher degree, and with the introduction of the orthochromatic or 

 isochromatic plates into micro-photography, the use of Bismarck- 

 brown has become superfluous. 



In fact, all the varieties of bacteria hitherto discovered may be 

 stained with solutions of the basic anilin colors, of course some 

 more or less quickly and more or less satisfactorily. Yet it is pos- 

 sible to strengthen the staining power of these dyes by special 

 methods, and thereby heighten their effect upon the bacteria. 



It is known that, in the process of staining, some substances are 

 able to play a peculiar mediating part between the coloring sub- 

 stance and the object to be stained. They are called mordants, and 

 have been in use for a long time, especially in the dyeing of cottons. 

 As the most important of them may be mentioned certain metallic 

 salts, in particular some combinations of lead, iron, and chrome, 

 and then also alum, tartar-emetic, and tannin. Several of them 

 have also been found useful for histological purposes for example, 

 acetate of alumina, which in combination with carmine has at- 

 tained great importance as alum-carmine. It is less adapted for 

 bacteriological investigation, being a pure nucleus stain and leav- 

 ing the bacteria almost uninfluenced. Yet in special cases, as 

 will be explained later, this particular stain has been utilized to 

 advantage. 



This is still more the case with another carmine combination, the 

 so-called picro-carmine, a compound of carmine and picric acid. 

 This compound color stains not only the nucleus, but also the body 

 of the cell and the connective tissue, in a peculiar manner, and is 

 therefore in a high degree suitable for differentiaing the constitu- 

 ent parts of tissues. The bacteria, however, as we have seen, are 

 not stained by picro-carmine. 



