ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 681 



ever, counterbalanced by considerable defects. The sharpness of tho 

 image is impaired by the threefold reflection, which is effected partly 

 by mirrors silvered at the back, and partly by the transparent mirror, 

 the two surfaces of which produce images which of course do not 

 coincide. By using still thinner mirrors this defect might be lessened 

 but not removed entirely. The right and left sides of the object are 

 inverted (though the image is otherwise erect), and this renders it 

 difficult to a most tiresome degree for the microscopist, who is 

 accustomed to the inverted motion of the object, to adjust it, 

 and still more to afterwards correct complicated drawings by 

 the ordinary microscopical image. Employing an orthoscopic 

 eye-piece or inverting the drawing arrangements is of no use, as 

 the microscopical image, compared with the drawing projected by 

 the camera, appears in both cases with right and left hand parts 

 interchanged. 



The camera, moreover, will not bear the application of the blue 

 glass disks supplied with the Oberhauser instrument for modifying 

 the light, as the image becomes almost invisible. As its character- 

 istic, however, is the relatively great brightness of the surface 

 of the paper, a smoked glass mirror, in place of the plain one s 1 , 

 would be the more serviceable arrangement, but the instrument is 

 not constructed so as to allow such a change to be readily made. 



The combination of lenses for the purpose of reducing the image 

 is, the author thinks, a valuable addition. The insertion of the 

 camera lucida is of course equivalent to lengthening the tube of 

 the Microscope, and the image is strongly magnified, often too much 

 so. It is only to be regretted that when both plano-convex lenses 

 are employed simultaneously, the image, already obscure, becomes 

 still less clear, and in some cases almost invisible. Dr. Cramer also 

 draws attention to the fact, not thought of by Hofmann himself, that 

 his camera lucida combined with the reducing apparatus, when 

 inserted in the tube of the Microscope instead of the eye-piece, will 

 give an image without the objective. The amplification with the 

 two lenses is about four times. He considers that " if Hartnack 

 could prevail upon himself to construct his camera lucida in such a 

 way that in the short arm, or in place of it, a combination of lenses 

 analogous to Hofmann's were introduced so that an image magnified 

 only four to eight times could be obtained, the value of this instru- 

 ment, already so desirable for the microscopist, would be materially 

 increased." 



Dr. Cramer then describes an instrument suggested by himself : — 

 " Those who use the Microscope, especially beginners, are not always 

 in a position to buy a camera lucida. I think, therefore, that I shall 

 be doing many a service by showing how any one who possesses a 

 little mechanical dexterity may make for himself the very serviceable 

 camera lucida shown in Fig. 125. 



" It consists essentially of two somewhat diverging mirrors, one of 

 which, S, allows the image of the object to be viewed direct through 

 a circular hole made by removing the quicksilver from the under side 

 of the mirror. By the second mirror, S\ the rays from the pencil and 



