EXPLANATION OF PLATE IV. 



Figures showing the use of the Abbe Camera Lucida (%% 122-125). 



Fig. 30. Abbe Camera Lucida with the mirror at 45 , the drawing surface hori- 

 zontal, and the microscope vertical. 



Axis, Axis. Axial ray from the microscope and from the drawing surface. 

 A B. Marginal rays of the field on the drawing surface, ab. Sectional view of the 

 silvered surface in the lower of the triangular prisms composing the cubical prism 

 (P). The silvered surface is shown as incomplete in the center, thus giving pas- 

 sage to the rays from the microscope. 



foot. Foot or base of the microscope. 



G. Smoked glass seen in section. It is placed between the mirror and the prism 

 to reduce the light from the drawing surface. 



Mirror. The mirror of the camera lucida. A quadrant (Q) has been added to 

 indicate the angle of inclination of the mirror, which in this case is 45 . 



Ocular. Ocular of the microscope over which the prism of the camera lucida is 

 placed. 



P, P. Drawing pencil and the cubical prism over the ocular. 



Fig. 31. Geometrical figure showing the angles made by the axial ray with the 

 drawing surface and the mirror. 



A B. The drawing surface. 



Fig. 32. The Abbe Camera Lucida with the mirror at 35 , and the position of the 

 drawing surface to avoid distortion ($ 124). 



Axis, Axis. Axial ray from the microscope and from the drawing surface. 



A B. Drawing surface raised toward the microscope 20 . 



Foot. The foot or base of the microscope. 



Mirror with quadrant (Q). The mirror is seen to be at an angle of 35 . 



Ocular. Ocular of the microscope. 



P, P. Drawing pencil, and the cubical prism over the ocular. 



W. Wedge to support the drawing board. 



Fig. 33. Geometrical figure of the preceding, showing the angles made by the 

 axial ray with the mirror and the necessary elevation of the drawing board to avoid 

 distortion. From the equality of opposite angles, the angle of the axial ray reflect- 

 ed at 35 must make an angle of no° with a horizontal drawing board. The board 

 must then be elevated toward the microscope 20 in order that the axial ray may 

 be perpendicular to it, and thus fulfill the requirements necessary to avoid distor- 

 tion (§$ 120, 124). 



Fig. 34. This shows the arrangement of the drawing surface for a mirror at 35 

 and the microscope inclined 30 (Mrs. Gage). ($ 125). 



Fig- 35- Upper view of the prism of the camera lucida. A considerable portion 

 of the face of the prism is covered, and the opening in the silvered surface appears 

 oval. 



Fig. 36. Ocular, showing eye-point, E P. It is at this point both horizontally 

 and vertically that the hole of the silvered surface should be placed ($ 123). 



Fig- 37- Quadrant to be attached to the mirror of the Abbe Camera Lucida to 

 indicate the angle of the mirror. As the angle is nearly always at 45 , 40 or 35 , 

 only those angles are shown. 



