H2 DRAWING WITH THE MICROSCOPE. [CH V. 



102, ab). The upper surface of the prism is covered by a perforated 

 metal plate (Fig. 108). This prism is placed over the ocular in such a 

 way that the light from the microscope passes through the hole in the 

 silvered face and thence directly to the eye. Light from the drawing 

 surface is reflected by a mirror to the silvered surface of the prism and 

 reflected by this surface to the eye in company with the rays from the 

 microscope, so that the two fields appear as one, and the image is seen 

 as if on the drawing surface. (Figs. 102, 106). It is designed for use 

 with a vertical microscope, but see § 174. 



§ 173. Arrangement of the Camera Lucida Prism. — In placing 

 this camera lucida over the ocular for drawing or the determination of 

 magnification, the center of the hole in the silvered surface is placed in 

 the optic axis of the microscope. This is done by properly arranging 

 the centering screws that clamp the camera to the microscope tube or 

 ocular. The perforation in the silvered surface must also be at the level 

 of the eye-point (Fig. 104). In other words, the prism must be so 

 arranged vertically and horizontally that the hole in the silvered surface 

 will be in the axis of the microscope and co-incident with the eye-point 

 of the ocular. If it is above or below, or to one side of the eye-point, 

 part or all of the field of the microscope will be cut off. As stated 

 above, the centering screws are for the proper horizontal arrangement 

 of the prism. The prism is set at the right height by the makers for 

 the eye-point of a medium ocular. If one desires to use an ocular with 

 the eye-point farther awaj' or nearer, as in using high or low oculars, 

 the position of the eye-point may be determined as directed in § 55 and 

 the prism loosened and raised or lowered to the proper level ; but in 

 doing this one should avoid setting the prism obliquely to the mirror. 



In the latest and best forms of this camera lucida special arrangements 

 have been made for raising or lowering the prism so that it may be used 

 with equal satisfaction on oculars with the eye-point at different levels, 

 and the prism is hinged to turn aside without disturbing the mirror. 



See the latest catalogs of Zeiss, L,eitz, and the Bausch & L,omb Op- 

 tical Co. 



One can determine when the camera is in a proper position by looking 

 into the microscope through it. If the field of the microscope appears 

 as a circle and of about the same size as without the camera lucida, then 

 the prism is in a proper position. If one side of the field is dark, then 

 the prism is to one side of the center ; if the field is considerably smaller 

 than when the prism is turned off the ocular, it indicates that it is not 

 at the correct level, i. <?., it is above or below the eye-point. 



§ 174. Arrangement of the Mirror and the Drawing Surface. — 



