CH. V.] 



DRA WING WITH THE MICROSCOPE. 



designed to be used on the microscope in a vertical position. As in bio- 

 logical work, it is often necessary to have the microscope vertical, the 

 form for a vertical microscope is to be preferred ; but see Figs. 102-1 1 1 . 



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

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

 axial ray from the image on the draiving surface shall be at right angles 

 to the drawing surface ("Figs. 102, 105, 106). 



§171. Wollaston's Camera Lucida. — This is a quadrangular prism of glass put 

 in the patli of the rays from the microscope, and it 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 (Fig. 105), the fields 

 partly overlap, and where they do so overlap, pencil or dividers and microscopic 

 image can be seen together. 



In drawing or using the dividers with the Wollastou camera lucida it is necessary 

 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 image will be very defi- 

 nite, but the pencil or dividers will not be visible. It is necessary, as with the 

 Abbe camera lucida (§ 173), to hav.e the Wollastou prism properly arranged with 

 reference to the axis of the microscope and the eye-point. If it is not, one will be 

 unable to see the image well, and may be entirely unable to see the pencil and the 

 image at the same time. Again, as rays from the microscope and from the draw- 

 ing 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 1 ell the proper position by trial. This is not a very sat- 

 isfactory 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 determin- 

 ing magnification ($ 153). 



Fig. 105. Wollaston's Camera Lu- 

 cida, showing the rays from the mi- 

 croscope and from the drawing sur- 

 face, and the position of the pupil of 

 the eye. 



For full explanation see Fig. 92. 



§ 17.2. Abbe Camera Luci- 

 da. — This consists of a cube of 

 glass cut into two triangular 

 prisms and silvered on the upper 

 one. A small oval hole is then 

 cut out of the center of the sil- 

 vered surface and the two prisms 

 are cemented together, thus giv- 

 ing a cubical prism with a per- 

 forated 45 degree mirror (Fig. 



