HISTOLOGICAL SUBJECTS. 207 



piece, I am not now prepared to speak ; most of my work has 

 been done without one. The eye-piece has, or should have, the 

 merit of rendering the field aplanatic. 



For light, kerosene lamps, such as are used with magic-lanterns, 

 etc., furnish sufficient for low powers, if the object presents strong 

 contrasts and sharp outlines, so that the definition is not taxed 

 too much. The flat wicks are preferable, although a round one 

 answers pretty well. With high powers, or where fine definition of 

 delicate details is needed, the oil-lamp is not sufiicient, and a 

 stronger light must be used — such as an arc electric light, or sun- 

 light. The strong, clear, white light of these two gives the best 

 definition, and also enables one to focus much more accurately. 

 Sunlight I have found all that could be desired. It is the most 

 available for general use, and in the United States National 

 Museum, after many trials of various other kinds of light, it is 

 considered the best for photo-micrographic purposes. The only 

 apparatus necessary in using it is a heliostat to reflect the rays 

 upon a condenser. The condensing lens should be about three 

 inches in diameter, and should have a focal length of from six to 

 eight inches. A slight variation in the size of the condenser makes 

 no material difference, provided it is not too convex, as the rays 

 entering the objective should be as nearly parallel as possible, in 

 order to secure more accurate definition. 



A substage condenser is not necessary for tissue photography, 

 except with very high powers. The size of the camera may be 

 left to individual choice. In the National Museum the one which 

 I used was suited for an eight-by-ten plate, and had a bellows 

 arrangement five feet long. If the eye-piece is used, two feet of 

 bellows will be sufficient. The plate-holder should be capable of 

 carrying a collodion plate, and also be provided with mats for 

 holding smaller sizes, if an eight-by-ten plate is generally used. 



If sunlight is used, the work-room should have one window 

 facing the south, the lower sash replaced by wooden shutters and 

 the upper one supplied with orange-coloured glass, and a thick 

 roller shade, by means of which the room may be darkened while 

 an exposure is being made, so that no actinic light will enter the 

 objective except that which passes through the condenser. 



The different parts of the apparatus are conveniently arranged 



