5 o ME THODS OF EX A MINA TION. 



for any adjustment, except that on the microscope, is done away with. 

 Even an ordinary argand gas-burner gives sufficient light for systems 

 magnifying up to about 400 diameters. 



In working with high powers it is necessary to concentrate the light 

 as much as possible, for which purpose two condensers are used — 

 one to bring the rays parallel, and a second to bring them to a focus 

 on the mirror, or directly on the specimen. 



All that is necessary is a flat board, to which the camera is 

 screwed (an ordinary wooden camera is quite good enough). Around 

 the aperture in the front of the camera is a cloth tube with an 

 elastic ring, which fits over the eye-piece of the microscope. 

 The microscope is clamped by means of thumb-screws to a 

 piece of board firmly fixed at right angles to that on which the 

 camera rests, and so far away that when the tube is pushed in it 

 leaves the projecting ring or shoulder of the eye-piece tube within the 

 elastic band. This apparatus may be used in a horizontal position, 

 where the specimens to be photographed are firmly fixed, as in the 

 case of tissues mounted in balsam, in which position the light is most 

 readily managed, but by simply having a couple of hooks at the 

 camera end of the board, the whole apparatus may be hung against 

 a wall, and the slide is kept perfectly horizontal, in which position 

 fluids or unfixed specimens may be easily photographed. One great 

 advantage connected with this apparatus is that as the specimen can 

 be searched, and the exact point to be photographed fixed before 

 the microscope is clamped to the foot-board, no mechanical stage 

 is necessary. The best photographs are obtained of specimens that 

 have been stained brown, hence, as Koch suggests, Bismarck brown, 

 vesuvin, or chrysoidin should be used as the staining reagents, 

 fairly good results, however, are frequently obtained from un- 

 stained preparations, and even from preparations stained with 

 eosin, or others of the aniline colours ; and as certain micro- 

 organisms can only be stained with actinic colours, contrasts must 

 often be called to our aid, double-staining being especially useful 

 in such cases. 



The mounting reagent must be as free from colour (yellow) as 

 possible ; this is of even greater importance than absolute transparency. 

 The best medium is a saturated solution of acetate of potash ; after 



