106 THE VICTORIAN NATURALIST. 



has also a scale of inches marked on the base board, so that one 

 may keep a record of the length of extension employed. These 

 are desiderata for the work, although an ordinary camera of any 

 size can be utiUzed for low power work, and be capable of pro- 

 ducing good results. 



The next important subject is the illuminant. Daylight is 

 seldom used, owing to its uncertain intensity ; limehght is more 

 generally employed by experienced photo-micrographers, the 

 advantage of which lies in its brilliancy and constancy. If a 

 small oil lamp, such as I have here, be used, there are some 

 objects — as bacilli, for instance — which, when enlarged by means 

 of a high power objective,- appear very dim on the ground glass of 

 the extended camera, and are consequently very difficult to focus, 

 whereas with the limelight they would be easily distinguishable. 

 However, for the sake of convenience we will restrict our atten- 

 tion at present to the use of a microscopical oil lamp as 

 illuminant. An iron chimney with a small opening, such as that 

 provided in the pattern here, has several advantages over other 

 kinds. It is practically unbreakable, and almost all the rays 

 from it can be concentrated on to the object. If the small 3x1 

 slip should meet with an accident it can be very readily replaced. 



A large bull's-eye condenser should be used to concentrate the 

 light from the lamp, and where high powers are used and critical 

 work is to be done a substage condenser will also be essential 

 (an achromatic condenser of large numerical aperture is required 

 for producing the best results). 



Having now decided what instruments are necessary for the 

 work, and assuming that such small accessories as dry plates, 

 chemicals, dishes, sensitive paper, printing frames, &c., have been 

 procured, we will proceed to describe the processes connected 

 with photo-micrography. 



The first step will be to centre the substage condenser. This 

 needs to be very accurately done if our photographs are to be of 

 first-class quality. The smallest diaphragm of the condenser 

 should be inserted, and with a low power the body of the micro- 

 scope should be adjusted till the image of the diaphragm is seen 

 in the field. If it does not appear exactly central, the substage 

 centering screws must be turned till this result is obtained ; the 

 stop should then be removed. 



The next step will be to focus the light from the lamp upon 

 the object. Screw on a i-inch objective, displace the mirror and 

 arrange the microscope in a hne with the light in a horizontal 

 position ; then place an object on the stage and focus it, after 

 which the substage with condenser should be racked up and 

 down till the image of the flame is seen well defined in the centre 

 of the field. 



I may here mention that if the substage were alvvays used in 



