Appendix A: The Microscope and Its Optical Principles 147 



with one portion of the pupil, and from the drawing-paper with the 

 remainder. 



Some form of the Abbe camera lucida, however, is used by most 

 workers. It consists of a cap which is fitted immediately above the eye- 

 piece and which contains two right-angle prisms cemented together to 



FIG. 60. Simple Camera, Lucida. FIG. 61. Camera Lucida, Abbe. 



form a cube (Fig. 61). The lower one of the prisms is silvered along its 

 cemented surface although a small central opening is left through which 

 the object under the microscope may be viewed ; connected with the cap 

 is an arm which bears a mirror and this mirror may be so adjusted as to 

 reflect the image of the drawing-paper on the table on to the prisms from 

 one side. The prisms are so set that the silvered surface of the lower 

 one reflects this image upward to the eye of the observer which also, co- 

 incidently, is viewing the magnified image of the object through the hole 

 in the silvering. When proper adjustment of the light received from 

 object and paper respectively is made, a pencil point may be distinctly 

 seen when brought into the field of vision over the paper ; consequently, 

 the outline of the object may be accurately traced. The secret of success 

 in working with a camera lucida is to have the illumination in the two 

 fields properly balanced. Small screens of tinted glass are provided 

 with the instrument for such regulation. Low-power eyepieces should 

 be used. With the Abbe camera lucida the microscope may be used in 

 a vertical or in an inclined position. If the microscope stand is inclined, 

 the drawing-board upon which the paper rests must have the same in- 

 clination, or the outline when drawn will be distorted. Likewise, if the 

 mirror of the camera is at any other angle than 45 degrees, an adjust- 

 ment of the drawing-surface must be made ; in short, the axial ray of the 

 image and the drawing-surface must always be at right angles to pre- 

 vent distortion. This means that if the mirror is depressed below 45 

 degrees the drawing-surface must be tilted toward the microscope twice 

 as much as the mirror is depressed. For example, if the mirror is de- 

 pressed to 37 degrees (8 below 45 degrees), the drawing-board must be 

 tilted (raised) 16 degrees. When the camera is in proper position the 

 field of the microscope should appear at about the same size as without 



