49 MAGNIFICA TION AND DRA WING. 



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 sil- 

 vered 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 ap- 

 pear as one, and the image is seen as if on the drawing surface (Fig. 

 50). It is designed for use with a vertical microscope, but see § 124. 



§ 123. 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. 36). In other words, the prism must be so ar- 

 ranged vertically and horizontally that the hole in the silvered surface 

 will be co-incident with the eye -point of the ocular. If it is above or 

 below or to one side, part or all of the field of the microscope will be 

 cut off. As stated above, the centering screws are for the proper hori- 

 zontal arrangement of the prism. The prism is set at the right height 

 by thegnakers for the eye-point of a medium ocular. If one desires to 

 use an ocular with the eye-point farther away or nearer, as in using 

 high or low oculars (§ 36), the position of the eye-point may be deter- 

 mined as directed in § 36 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. 



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

 ing into the microscope through it. If the field of the microscope ap- 

 pears 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 con- 

 siderably smaller than when the prism is turned off the ocular (§ 124) 

 it indicates that it is not at the correct level, i. e., it is above or below 

 the eye-point. 



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

 The Abbe camera lucida was designed for use with a vertical micro- 

 scope (Fig. 30). On a vertical microscope, if the mirror is set at an 

 angle of 45 the axial ray will be at right angles with the table top or 

 a drawing board which is horizontal (Fig. 30), and a drawing made 

 under these conditions' would be in true proportion and not distorted. 

 The stage of most microscopes, however, extends out so far at the 

 sides that with a 45 mirror the image appears in part on the stage of 

 the microscope. In order to avoid this the mirror may be depressed to 

 some point below 45 , say at 40 or 35 (Fig. 32, 33). But as the axial ray 

 from the mirror to the prism must still be reflected horizontally, it fol- 





