ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 501 



cross-pieces may be used for the attachment of a variety of accessories. 

 Among the more important of these is the Microscope camera (see fig. 

 60, 19). This hangs above the Microscope, and is ever in readiness 

 for instant use. The camera itself presents no very peculiar features. 

 It is, of course, a vertical pattern, carrying the exposed photographic 

 plate in a horizontal position. It cannot be used in a horizontal posi- 

 tion. Experience has shown that the vertical position has very many 

 advantages, and that if one is confined to a single outfit, the vertical 

 outfit is the better, providing its attachment can be of the nature here 

 described. In obtaining the focus, the cross-piece carrying the camera 

 is loosened by unclamping the side screws, and is then moved upward 

 and downward against the sash-weights, which counterbalance it. A 

 scale is marked on the girders, so that the various magnifications are at 

 once obtainable, or they may be obtained by special measurement in 

 each case. The apparatus never needs any levelling, being, as before 

 said, constantly ready for use. The operator loosens two hooks, and 

 the camera drops instantly into position. The whole is ready for use in 

 a few seconds' time. If the photograph is being taken with a high 

 power, and the illumination is, therefore, weak, and the exposure conse- 

 quently long, one leaves his instrument during the exposure with the 

 greatest confidence that nothing can disturb it. Any tremors in the 

 building will not be received either by the Microscope or the photographic 

 plate. A second attachment of great importance for the production of 

 illustrations is the 



CAMEEA LUCIDA. 



This presents a number of peculiarities (fig. 60, 17, S4). 



Any form of camera-lucida is an instrument well calculated for the 

 destruction of eyesight. The writer has during many years of experi- 

 ence been endeavouring to reduce the injury to the eyesight in 

 connection with the use of the camera-lucida, and the following sug- 

 gestions, embodied in the outfit here described, are the result of his 

 experience. In the first place, he has substituted for the ordinary 

 mirror a 45° prism (fig. 60, 2jf). The advantages obtained by this 

 substitution are as follows : — (1) The prism may be of any desired size, 

 so that it may be mounted at a considerable distance from the eye-piece 

 of the Microscope. This secures an increased magnification of the 

 drawing, and the advisability of this increased magnification will be 

 dwelt upon on a subsequent page ; (2) a second advantage in the use of 

 the prism as a reflector is the disappearance of the double reflection, 

 and the securing of a total reflection. The light passes from the 

 drawing-point through the lower face of the prism in a nearly perpen- 

 dicular direction, and with very little loss. It is then totally reflected 

 from the oblique face, and passes outward at nearly right angles to the 

 vertical face, again with very slight diminution. The loss of light is, 

 therefore, considerably less than in the case of the usual mirror, in 

 addition to the securing of a total reflection destitute of doubles ; (3) a 

 third advantage, and one of considerable importance, is the stability of 

 the apparatus here described ; it rarely gets out of register. 



