CAMERA LUCID A. 



99 



through the Eye-piece into the Prism sustain such a total reflexion 

 from its oblique surface, that they come to its upper horizontal 

 surface at right angles to their previous direction ; and the eye 

 being so placed over the edge of this surface that it receives these 

 rays from the prism though part of the pupil, whilst it looks 

 beyond the prism down to a white paper surface on the table with 

 the other half, it sees the image so strongly and clearly projected 



Fig. 53. 



Microscope arranged with Camera Lucida for Drawing or Micrometry. 



upon that surface, that the only difficulty in tracing it arises from 

 a certain incapacity which seems to exist in some individuals for 

 seeing the image and the tracing-point at the same time. This 

 difficulty (which is common to all instruments devised for this pur- 

 pose) is lessened by the interposition of a slightly convex lens in 

 the position shown in the figure, between the eye and the paper, 

 in order that the rays from the paper and tracing-point may diverge 

 at the same angle as those which are received from the prism ; 

 and it may be generally got over altogether by experimentally 

 modifying the relative degrees of light received from the object 

 and from the paper. If the image be too bright, the paper, the 

 tracing-point, and the outline it has made, are scarcely seen ; and 

 either less light may be allowed to come from the object, or more 

 light (as by a taper held near) may be thrown on the paper and 

 tracing-point. Sometimes, on the other hand, measures of the 



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