PHOTOGRAPHY OF BACTERIA. 153 



results ; in others again, compared with the results of staining 

 with blue or red stains, there was much to be desired, and further 

 improvement was needed. 



That a stain, such as yellow or brown, must be employed which 

 absorbs the blue rays, and acts on the sensitive plate like black, 

 which absorbs all the light, constituted the first condition laid 

 down by Koch as an essential for success. It was further pointed 

 out that the suitability of the stain could be ascertained by first 

 passing the light, to illuminate the preparation, through a solution 

 of ammonio-sulphate of copper, under which condition the bacteria 

 would appear black on a blue ground. 



The second condition was, that sunlight must be employed, but 

 that direct projection upon the object was disadvantageous, and it 

 must, therefore, be diffused by the interposition of one or more plates 

 of ground glass. 



Lastly, an illuminating condenser was recommended, of such 

 construction that the diffused sunlight brightly illuminates the object 

 from all sides. 



Sternberg encountered the same difficulty in photographing red, 

 blue or violet preparations, while he produced excellent pictures of 

 preparations stained with aniline brown, or a weak solution of iodine 

 (iodine grs. iij, potassic iodide grs. v, distilled water grs. 200). Thus 

 the results of a large number of attempts to photograph the tubercle 

 bacillus in sputum, only ended in producing such extremely faint 

 impressions, that any one unacquainted with the object as seen under 

 the microscope could form scarcely any idea of its form or minute 

 structure with even an accompanying explanation and the closest 

 inspection of the photograph. 



Dufrenne, in attempting to photograph the same object by the 

 ordinary method, found the plates were uniformly acted on, or the 

 image was so faint, or so lacking in contrast, that they were useless 

 for obtaining proofs on paper or glass. By interposing green glass 

 between the objective and the sensitive plate, so that the red rays 

 were absorbed, while the green rays passed through and acted on 

 the plate, he states that better results were obtained. 



The work of Hauser illustrated the great value of photography 

 in the production of pictures of impression-preparations and colonies 

 in nutrient gelatine. To give the general effect, as well as faithfully 

 reproduce the minute details in these difficult subjects would in most 

 cases create insurmountable difficulties, except to the most accom- 

 plished draughtsman. 



Hauser employed Gerlach's apparatus and Schleussner's dry 



