SIMPLE PHOTO-MICROGRAPHY 85 



beUows and re-focus until the size is satisfactory. 

 Then we insert the dark slide and make the ex- 

 posure, which wiU, of course, vary according to the 

 degree of illumination and of the nature of the 

 object. The light can be cut off as required by 

 placing a card between the object and the objective. 



Artificial light is best for photo-micrography, it 

 being less variable than daylight. The worker will 

 discover the exposure required by experimentation. 

 Some writers on the subject recommend slow plates. 

 I find fairly rapid ortho chromatic plates work well, 

 and I generally use Imperial " Non-Filter," because 

 they provide a moderately rapid ortho chromatic 

 plate plus a light filter — a particularly valuable 

 feature in photo-micrographic work. There must not 

 be the slightest vibration of the apparatus during 

 exposure, and the worker will help to avoid this by 

 keeping quite still at the critical moment. 



It very often happens that a photograph is 

 desired of some object which requires only small 

 magnification, but which is of such a nature that an 

 ordinary microscopic objective will not cover it or 

 provide sufficient depth of focus. I have more than 

 once met this difficulty by using the ordinary camera 

 lens and photographing the object through a large 

 reading-glass. But results so obtained are not of 

 the most satisfactory kind ; the image is apt to be 

 distorted. Several manufacturing opticians have 

 faced this problem, and now lenses are to be obtained 

 which work at a wide aperture and have great 

 covering properties at short distance from the 

 object, and considerable depth of focus. They are 

 practically short-focus photographic lenses mounted 



