852 SUiniAEY OF current KE8EAECHES RELATING TO 



to be distinguislied on the photographic plate, even when the latter 

 was made very sensitive. Tincture of iodine and solutions of gold 

 and silver salts were also tried, with negative results. Finally the 

 cell-wall of the bacteria was subjected to a photo-chemical process in 

 the following manner : — 



After the drop containing the bacteria is dried upon the slide in 

 the usual manner, the spot is moistened with an aqueous solution of a 

 metallic iodide (cadmium iodide 1 : 50 was employed), and in two or 

 three minutes the bacteria are sufficiently iodized. The slide is then 

 carefully and rapidly washed with distilled water, and immediately 

 flowed with a few drops of silver solution from the negative-bath. If 

 the right time has been hit, and the iodide has not acted for too short 

 a period, and the washing has not been continued too long, the contour 

 of the dried drop will be seen to show a slight yellow colour, due to 

 iodide of silver, which is formed. Only an exceptionally short expo- 

 sure to light is sufficient, after which the developer (strongly acidified 

 and dilute iron-developer) is added, and the drop suddenly becomes 

 black. After thorough washing the deeply coloured bacteria are 

 mounted in balsam, and they may then be readily photographed. 



This method is only useful for photographic purposes, and there 

 is some chance of mistaking fine silver precipitates for micrococcus 

 or other forms. In case of any doubt of this kind, the original forms 

 should be stained with anilin colours, and examined in the usual way. 



Dr. G. M. Sternberg considers* this method of staining bacteria 

 for photographing an improvement upon Koch's method of staining 

 with anilin violet, for the violet gives very little photographic con- 

 trast, because it permits the actinic rays to pass. A method which 

 he has employed with success, and believes to be new, is the follow- 



The bacteria are dried upon a slide, or upon a thin glass cover, 

 and are then treated with commercial sulphuric acid, a drop of which 

 is placed upon them. Aft^r two or three minutes the acid is washed 

 off by a gentle stream of water, and the bacteria are then covered with 

 an aqueous solution of iodine (iodine grs. 3, potassic iodide grs. 5, 

 water grs. 500). After a few minutes they will be found to present 

 a deep orange or brown colour, which gives the desired contrast in a 

 photograph negative. 



This method is only useful for extemporaneous preparations which 

 are to be photographed immediately. The colour fades after a time, 

 and the bacteria undergo changes in form (swelling) as a result of 

 this treatment, which renders the method unsatisfactory when the 

 object is to make a permanent preparation. For this purpose nothing 

 is better than the anilin violet, which indeed leaves nothing to be 

 desired when a collection is being made without reference to photo- 

 graphy. The specimens should be mounted either in solution of 

 acetate of potash (Koch's method), or preferably in carbolic acid water. 



Anilin violet ink, which may be obtained from any stationer, is a 

 cheap and satisfactory staining fluid. One or two minutes' immersion 



♦ Amer. Mon. Jlicr. Joum., ii. (1881) pp. 86-7. 



