Ch. VI] DRAWING WITH A CAMERA LUCIDA 163 



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 fig. 99, 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 Wollaston camera lucida 

 it is necessary to have the field of the microscope and the drawing 

 surface about equally hghted. If the drawing surface is too bril- 

 liantly 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 definite, but the pencil or dividers wiU not be clearly visible. 

 It is necessary, as with the Abbe camera lucida (§ 271), to have the 

 Wollaston prism properly arranged with reference to the axis of the 

 microscope and the eye-point. If it is not, one wiU 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 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 

 teU 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 ver- 

 tical distance of 250 mm. at which the drawing surface should be 

 placed when determining magnification (fig. 85). 



§ 271. Abbe camera lucida. — This consists of a cube of glass 

 cut into two triangular prisms and silvered on the cut surfaee of the 

 upper one. A small oval hole is then cut out of the center of the 

 silvered surface and the two prisms are cemented together in the form 

 of the original cube with a perforated 45 degree mirror within it 

 (fig. loo-ioi). The upper surface of the cube is covered by a per- 

 forated metal plate. This cube is placed over the ocular in such a 

 way that the Ught from the microscope passes through the hole in 

 the silvered face and thence directly to the eye. Light from the 

 drawing surface is reflected by the mirror to the silvered surface of 

 the prism and reflected by this surface to the eye in company with 



