CAMERA LUCIDA. 



125 



Fig. 34. 



" cap"- by which it is usually surmounted. The Microscope being 

 placed in a horizontal position, as shown in Fig. 34, the rays 

 which pass through the eye-piece into the prism, sustain such a 

 total reflection 

 from its oblique 

 surface, that they 

 come to its upper 

 horizontal sur- 

 face at right an- 

 gles to their pre- 

 vious direction ; 

 andtheeyebeing 

 so placed over 

 the edge of this 

 surface, that it 

 receives these 

 rays from the 

 prism through 

 part of the pupil, 

 whilst it looks 

 beyond the 



nrism down to a *''''"'^°°P^ arranged with Camera Lucida for Drawing or Micrometry. 



white-paper surface on the table, with the other half, it sees the 

 image so strongly and clearly projected 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 purpose) 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 modi- 

 fying 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 contrary kind must be taken. Instead of the prism, some 

 microscopists prefer a speculum of polished steel, of smaller size 

 than the ordinary pupil of the eye, fixed at an angle of 45° in 

 front of the eye-piece ; and this answers exactly the same purpose 

 as the preceding, since the rays from the eye-piece are reflected 

 vertically upwards to the central part of the pupil placed above 

 the mirror, whilst, as the eye also receives rays from the paper 

 and tracer, in the same direction, through the peripheral portion 

 of the pupil, the image formed by the microscope is visually pro- 

 jected downwards, as in the preceding case. This disk, the in- 



