METHODS OF INVESTIGATING BACTERIA. 787 



The appearances observed under the microscope are Photography 

 best reproduced by means of photography. The cover of bactena> 

 glass preparations prepared in the manner above de- 

 scribed permit the employment of high power immersion 

 lenses. It is best to stain the organisms brown. 

 Photography also possesses a special advantage in that 

 the photographic plate reproduces the microscopic 

 picture better, or rather with greater accuracy, than does 

 the retina of the eye. 



The sensitive plate is to a certain extent an eye which is 

 not blinded by bright light, and which does not become tired 

 by the attempt to distinguish between minute differences in 

 light, &c. We often find 011 the negative, when the picture 

 has been sharply focussed, fine objects, for example, minute 

 nagella, which can only be seen with the greatest difficulty, 

 and under the most favourable conditions of illumination. 



As regards the methods of photography see the works 

 mentioned below.* 



Bacteria, more especially micrococci, may be con- Differential 

 founded with detritus, but the latter shows granules 

 irregular size and grouping ; we also at times find small 

 drops or little spheres, which become stained with the 

 dyes, and the nature of which is as yet doubtful. Con- 

 fusion is most likely to occur with the cells described by 

 Ehrlich under the name of " Mastzellen " (plasma cells, 

 granular cells), which are very widely distributed, and 

 increase in number in various pathological processes. 

 The uniform round granules of these cells are usually 

 stained in a similar manner, and with a similar shade, 

 to the bacteria ; and the two forms can often be only 

 distinguished by the position which they occupy, and 

 more especially by the fact that the granules in the 

 plasma cells are always grouped in the form of cell-like 

 structures. If it is necessary to make certain that we 

 are studying bacteria the following method may be 

 employed : after staining with aniline the sections are 



* Gerlach, Die Photographic als H&lftmittel mikroskoplscher Forschung 

 Leipzig, 1863. Beneke, Die P hoi oy rap hie als Hillfsmittel u. s. w., Braun- 

 chweig, 1868. Eeichardt u. Stiirenburg, Lehrbuch der mikroskopischen 

 Fhntographie, Leipzig, 1868. Vogel, Lehrbuch der Photoc/raphie, Berlin, 

 1874. See also the excellent photographs by E. Koch in Cohn's Seitr., 

 vol. 2, and in the Mittheilunyen a. d. Kais. Gesundheits-Amt., vol. 1. 



