PHOTOGRAPHING BACTERIA. 195 



must be done with high powers, and the technical 

 difficulties to be overcome are by no means incon- 

 siderable. The illustrations in the present volume 

 may be taken as fair samples of what may be ac- 

 complished, and it will be found easier to criticise 

 these than to improve upon them. Koch says, in 

 his " Traumatic Infective Diseases " : 



" In a former paper I expressed the wish that ob- 

 servers would photograph pathogenic bacteria, in order 

 that representations of them might be as true to nature 

 as possible. I thus felt bound to photograph the bac- 

 teria discovered in the animal tissues in traumatic infec- 

 tive diseases, and I have not spared trouble in the 

 attempt. The smallest, and in fact the most interesting, 

 bacteria, however, can only be made visible in animal 

 tissues by staining them, and by thus gaining the ad- 

 vantage of color. But in this case the photographer 

 has to deal with the same difficulties as are experienced 

 in photographing colored objects, e. g., colored tapestry. 

 These have, as is well known, been overcome by the 

 use of colored collodion. This led me to use the same 

 method for photographing stained bacteria ; and I have 

 in fact succeeded, by the use of eosin-collodion, and by 

 shutting off portions of the spectrum by colored glasses, 

 in obtaining photographs of bacteria which had been 

 stained with blue and red aniline dyes. Nevertheless, 

 from the long exposure required and the unavoidable 

 vibrations of the apparatus, the picture does not have 

 sharpness of outline sufficient to enable it to be of use 

 as a substitute for a drawing, or indeed even as evi- 

 dence of what one sees. For the present, therefore, 

 I must abstain from publishing photographic repre- 

 sentations." 



