PHOTOGRAPHY OF BACTERIA. 163 



either candles, oil, or gas. In any case, the light must pass through 

 two thicknesses of ruby glass. 



Dry Plates. — A small supply of any of the ordinary plates in 

 the market may be procured for preliminary trials in acquiring a 

 knowledge of the processes ; but to overcome the difficulties of certain 

 stained preparations, the isochromatic or orthochromatic plates should 

 be used. The | plate will be found to be the most suitable size. 



There are numerous formulae for the requisite solutions for 

 developing and fixing the negatives, and instructions are usually 

 enclosed in the boxes of dry plates, but it is best to abstain from 

 trying a number of different formulae, as it leads to a great expendi- 

 ture of time. There is a temptation to do this, it being supposed 

 that there is probably some great advantage in one formula over 

 another. It is much better to get accustomed to the behaviour 

 under different exposures of one, or perhaps two methods. 



In France the ii'on developer is much in vogue, and is recom- 

 mended by Tailfer and Clayton for use with their isochromatic 

 plates. It has the advantage of great simplicity in the mode of 

 employment, and, therefore, is very suitable for a beginner. In 

 England, on the other hand, the alkaline developer is very 

 much used, as it gives more command over the plate, enabling the 

 photographer more fully to compensate for incorrect exposure. 



It is very desirable before attempting to take photographs with 

 the microscope to learn how to take photographs with an ordinary 

 landscape camera, and to get thoroughly accustomed to the use of 

 some good developer, so that mistakes may be corrected and the 

 clearest and sharpest negatives obtained. 



Practical Manipulation. 



Arrartgement of Appm-atus. — For working with the parafiine 

 lamp, the mode of procedure is, as regards the illumination, briefly 

 as follows. The sub-stage condenser is dispensed with when a 

 low power is employed, as well as the mirror, and the lamp is 

 so placed that the image of the flat of the flame appears accurately 

 in the centre of the field of the microscope. A bull's-eye condenser 

 is then interposed, so that the image of the flame disappears, and 

 the whole field is equally illuminated. With high powers the 

 sub-stage achromatic condenser is necessary, and a more intense 

 illumination is obtained by using the flame edgewise. In using a 

 low power with the oxyhydrogen light, the lantern is withdrawn 

 some little distance from the microscope, and the top combination 

 of the achromatic condenser removed. 



