MICROSCOPY. 365 



and erythrusine. A trial can be made on Bacterium termo, 

 which is one of the most difficult of preparation. It is easi- 

 ly procured by exposing a piece of raw meat, placed with 

 water in a porcelain cup, to the sun for an hour or two, or 

 letting it stand near a warm oven. When an opal-like scum 

 has formed on the fluid, every drop is seen under the micro- 

 scope to contain millions of these bodies. This, or any other 

 fluid containing Bacteria (urine, serum, blood, etc.), should 

 be put in a 10-gram glass which has been carefully wash- 

 ed and rinsed in alcohol. The bottle should be a fourth or 

 a fifth part filled, and the same quantity of a solution in wa- 

 ter (well filtered) of the staining material added, and then, 

 after being well shaken, corked and labelled. From five or 

 ten minutes to forty-eight hours are required, according to 

 the nature of the object. When examination with the micro- 

 scope shows the result is satisfactory, a drop is taken from 

 the bottom by means of a pipette and spread out well on a 

 glass slide, and dried in a warm place, protected from dust. 

 A drop of dammar varnish, or Canada balsam, is applied, 

 the covering-glass is pressed down, and the preparation is 

 ready for examination, and may be preserved indefinitely 

 {Zeitschrift fur Mikroskopie, vol. i., p. 175). 



Examining", Preserving', and Photographing Bacteria. 



The principal difficulties which arise in investigating Bac- 

 teria are connected with their small size, their movements, 

 their simplicity of form, and their want of color or power of 

 strongly refracting light. Dr. Koch proposes, in order to ob- 

 viate these difficulties, the following process : A drop of the 

 fluid containing the Bacteria is spread out in as thin a film 

 as possible on the covering-glass and dried. In this condi- 

 tion they may remain for months, if kept from dust, etc., 

 without any change to the dried Bacteria ; and, as they dry 

 without shrinking or changing their form, there is no objec- 

 tion to this part of the process. The next step is to moisten 

 the film with acetate of potash (one part in two parts of dis- 

 tilled water) ; the Bacteria resume perfectly their original 

 form, and the fluid answers as a special preservative from 

 further change. They are yet too pale for photographing ; 

 for this purpose they must be stained with aniline dye, which 

 they take quickly and completely. Methyl-violet and fuchsine 



