152 BACTERIOLOGY. 



As these difficulties had been to a certain extent obviated by the 

 use of eosin-collodion, Koch adopted the same method for photo- 

 graphing stained bacteria. By the use of eosin-collodion, and by 

 shutting off portions of the spectrum by coloured glasses, he 

 succeeded in obtaining photographs of bacteria which had been 

 stained with blue and red aniline dyes. But, owing to the long 

 exposure which was necessary, and the unavoidable vibrations of 

 the apparatus, the results were so wanting in definition that they 

 not only proved unsatisfactory as substitutes for drawings, but did 

 not in some cases give any evidence of what was to be seen in the 

 preparations. 



Koch, in consequence, stated that he would abstain from 

 publishing photographic illustrations until he had the advantage 

 of improved methods. 



We find, however, in spite of this, that in 1881 Koch published 

 a, series of reproductions from his negatives in illustration of what 

 could be accomplished by photography. 



Here again we find that many of the photographs of cover- 

 glass preparations were admirable, but those of tissue-sections gave 

 evidence of the difficulties Koch encountered, and were undoubtedly 

 unsatisfactory from the want of flatness of field, some of the 

 illustrations recalling rather a map of a mountainous country than 

 a microscopical preparation. 



In consequence of the difficulties met with in attempting to 

 photograph bacteria stained with the aniline dyes most commonly 

 used, Koch recommended that the preparations should be stained 

 brown, pointing out as his reason that, though the bright and 

 concentrated colour of the red and blue aniline dyes catches the 

 eye far more readily than the somewhat sombre brown colours, 

 yet no one up to the time of his publication had succeeded in 

 obtaining good photographs of bacteria which had been stained 

 either blue or red, and mounted in Canada balsam, while there was 

 no difficulty in obtaining photographic representations of prepara- 

 tions stained yellow or brown. 



Though this stain could be easily employed in most cover-glass 

 preparations, it was by no means easy to obtain a good differential 

 stain of bacteria in the tissues by employing Bismarck brown. 

 An attempt was, therefore, made to photograph preparations 

 stained blue and red by the aid of the dry-plate process, and by 

 interposing glasses of suitable tints. After many fruitless experi- 

 ments this method had to be abandoned, and the method of staining 

 the object brown was adopted. In many cases this gave excellent 



