ACCESSORY APPARATUS. 97 



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 contrary kind must be taken. Another in- 

 strument for the same purpose, invented by the celebrated anatomist 

 Soemmering, and preferred by some Microscopists, is a flat speculum of 

 polished steel or speculum-metal, of smaller diameter than the ordinary 

 pupil of the eye, fixed at an angle of 45 in front of the eye-piece. 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 



Chevalier's Camera Lucida. Nachet's Camera Lucida. 



portion of the pupil, the image formed by the Microscope is visually pro- 

 jected downwards. In another form of Camera Lucida, devised by Amici, 

 and adapted to the horizontal microscope by Chevalier, the eye looks 

 through the Microscope at the object (as in the ordinary view of it), 

 instead of looking at its projection upon the paper, the image of the 

 tracing-point being projected upon the field an arrangement which is in 

 many respects more advantageous. This is effected by combining a per- 

 forated steel mirror with a reflecting prism; and its action will be under- 

 stood by the accompyaning diagram (Fig. 69). The ray a b proceeding 

 from the object, after emerging from the eye-piece of the Microscope, passes 

 through the central perforation in the oblique mirror M, which is placed 

 in front of it, and so directly onwards to the eye. On the other hand, 

 the ray a' proceeding upwards from the tracing-point, enters the prism p, 

 is reflected from its inclined surface to the inclined surface of the mirror 

 M, and is by it reflected to the eye at &', in such parallelism to the rap # 

 proceeding from the object, that the two blend into one image. The 

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