48 MA GNIFICA TION AND DRA II 7 ING. 



paper, pencils, etc. In some of the camera lucidas of this group (Wollaston's 

 PI. Ill, Fig. 27), the rays are reflected twice and the image appears as when look- 

 ing directly into the microscope. In others, the rays are reflected hut once and 

 the image has the inversion produced by a plane mirror. For drawing purposes 

 this inversion is a great objection, as it is necessary to similarly invert all the de- 

 tails added free-hand. 



(B) By a camera lucida reflecting the rays of light from the drawing paper, etc., 

 so that their direction when they reach the eye coincides with the direction of the 

 rays from the microscope (PI. IV, Fig. 30, 32). In all of the camera lucidas of 

 this group, the rays from the paper are twice reflected and no inversion appears. 



The better forms of camera lucidas (Wollaston's, Grunow's, Abbe's, etc.), may 

 be used for drawing both with low and with high powers. Some require the mi- 

 croscope to be inclined (Fig. 27), while others are designed to be used on the 

 microscope in a vertical position. As in biological work it is often necessary to 

 have the microscope vertical, this form is to be preferred. [ (B. 31, 355), (Beh. 

 no), (C. 112), Car. 73), (J. R. M. S. nearly every volume). For drawing at a mag- 

 nification of from 5 to 100 diameters for large objects see (W. 132), (J. R. M. S. 

 1881, p. 819; 1882, p. 402; 1884, p. 115; 1888, p. 113), (Amer. Naturalist, 18S6, p. 

 1071; 1887, pp. 1040, 1043)]. 



§ 120. Avoidance of Distortion. — In order that the picture drawn 

 by the aid of a camera lucida may not be distorted, it is necessary that 

 the axial ray from the image on the drawing surface shall be at right 

 angles to the drawing surface (PI. Ill, Fig. 27, PI. IV). 



\ 121. Wollaston's Camera Lucida. — This is a quadrangular prism of glass put 

 in the path of the rays from the microscope, and serves to change the direction of 

 the axial ray 90 degrees. In using it the microscope is made horizontal, and the 

 rays from the microscope enter one half of the pupil while rays from the drawing 

 surface enter the other half of the pupil. As seen in the figure (PI. Ill, Fig. 27) 

 the fields party overlap, and where they do so overlap, pencil or dividers and mi- 

 croscopic image can be seen together. 



In drawing or using the dividers with the Wollaston camera lucida it is neces- 

 sar} r to have the field of the microscope and the drawing surface about equally 

 lighted. If the drawing surface is too brilliantly lighted the pencil or dividers 

 may be seen very clearly but the microscopic image will be obscure. On the 

 other hand, if the field of the microscope has too much light the microscopic im- 

 age will be very definite, but the pencil or dividers will not be visible. Again, as 

 rays from the microscope and from the drawing surface must enter independent 

 parts of the pupil of the same eye, one must hold the eye so that the pupil is 

 partly over the camera lucida and partly over the drawing surface. One can tell 

 the proper position by trial. This is not a very satisfactory camera to draw with, 

 but it is a very good form to measure the vertical distance of 250 mm. at which 

 the drawing surface should be placed when determining magnification ($ 104). 



§ 122. Abbe Camera Lucida. — This consists of a cube of glass cut 

 into two triangular prisms and silvered on the lower one. A small oval 

 hole is then cut out of the center of the silvered surface and the two 

 prisms are cemented together, thus giving a cubical prism with a perfor- 

 ated 45-degree mirror (PI. IV, Fig. 30, a b). The upper surface of the 

 prism is covered by a perforated metal plate (Fig. 35). This prism is 

 placed over the ocular in such a way that the light from the microscope 



