HISTOLOGICAL SUBJECTS. 215 



normal working distance of the objective. A good negative may 

 be enlarged, to bring the details of a convenient size, without 

 losing definition. 



The high-power objectives are absolutely essential for bacteria, 

 but in this case a large field is not especially necessary, nor is the 

 same amount of })enetration required as in a section of tissue, 

 with a variety of details to be clearly defined. The sections used 

 in photo-micrography must be cut with a microtome, and must be 

 thoroughly good in every respect if it is desired to obtain a good 

 picture. 



My experience with the different stains is limited. Carmine 

 seems to work well, from a very delicate pink to a deep-red colour. 

 Hcematoxylin, glycerine, and nitrate of silver have all proved 

 satisfactory. The only case that gave me any trouble was a rather 

 thick section, deeply stained with a dark Bismarck-brown. The 

 cell bodies vrere of a very brown-yellow, and obstructed the light 

 effectually with botli collodion and dry plates, so that there was no 

 differentiation of the nuclei or other details. 



Polarised light with crystals gives most brilliant photo-micro- 

 gra])hs. The alkaloids are easily prepared, and, perhaps, present 

 fewer difficulties for one to begin with than tissues. The polari- 

 scope is put upon the substage as usual, and the analyser screwed 

 into the tube just above the objective. The blue cell is not 

 necessary. 



Positives or prints may be taken on albuminised paper or on 

 the non-albuminised. The latter, if well done, gives very pretty 

 effects in grey and white. The paper should be freshly sensitised 

 as it is needed, and, when dried, put into a box over fumes of 

 strong ammonia (to counteract any acidity wliich may be developed 

 during the toning) for twenty minutes, and then laid away smoothly 

 in a paper bag in a cool, dry, and i)erfectly dark place until 

 required. 



The printing should be carried on until the details are well 

 marked, and a little darker than they arc desired to be after the 

 photograph is finished. The ordinary toning and fixing baths may 

 be used. The prints on albuminised paper, when mounted, 

 .should be burnished in order to secure the best results. Prints on 

 non-albuminised paper do not need to be burnished, but afford an 



