CH. V~\ DRA WING WITH THE MICROSCOPE 143 



directly into the microscope. In others the rays are reflected but 

 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 details 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 (Fig. 

 65, 124). 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 microscope to be inclined (Fig. 

 in) 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, the form for a vertical microscope is 

 to be preferred ; but see Fig. 1 30. 



§ 200. Avoidance of Distortion. — In order that the picture 

 drawn by the aid of a camera lucida may not be distorted, it is neces- 

 sary that the axial ray from the image on the drawing surface shall be 

 at right angles to the drawing surface (Figs. 127, 129). 



§ 201. Wollaston's Camera Lucida. — This is a quadrangular prism of 

 glass put in the path 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 

 figure 127, the fields partly overlap, and where they do so overlap, pen- 

 cil 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 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 definite, but the pencil or dividers will not be 

 visible. It is necessary, as with the Abbe camera lucida ( \ 203 ) , to have the 

 Wollaston prism properly arranged with reference to the axis of the micro* 

 6Cope 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 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 dis- 



