62 Modern Microscopy. 



hospitals, etc., it is usual to supply one of these with nearly 

 every instrument. These are now to be had made of an 

 aluminium alloy which is extremely light, reducing the 

 strain on the body tube. 



Camera Lucida. 



This is designed to assist in drawing objects seen in 

 the microscope. Photo-micrography has to a large extent 

 superseded it ; still, there are a great many who prefer 

 this method to any other. Dr. Beale's neutral-tint reflector, 

 which is supplied by all the opticians, is the cheapest and 

 a very good form. The microscope is set in a horizontal 

 position, with the centre of the eyepiece 10 inches from 

 the table. Illumination is arranged in the ordinary way. 

 The cap of the eyepiece is removed, and the camera lucida 

 placed on so that the neural-tint glass is exactly opposite 

 the eye-lens of the eyepiece at an angle of 45. On look- 

 ing on to this neutral-tint glass from the upper side, a 

 disc of bright light will be seen on it, and if a piece 

 of white paper be spread below on the table, on further 

 examination the outlines of the object will be discerned. A 

 pencil should now be taken, and the specimen can be 

 sketched in its magnified form. This will be found some- 

 what difficult at first, nearly every worker having a different 

 method, but the secret is to arrange the balance of illumina- 

 tion by turning the lamp-wick up and down until a degree 

 of light is found at which the pencil-point and image can 

 be distinctly seen. Of much more expensive description, 

 but considered the best at present made, is the Abbe camera 

 lucida (Fig. 23). The instrument need not be set in a 

 horizontal position ; in fact, the microscope may be at any 

 angle, and this may still be employed. It consists of a 

 mirror which reflects the paper to be drawn on through 

 a right-angle prism having a silvered surface into the field 

 of view. This prism is fitted over the eyepiece, and by 



