124 DRAWING WITH THE MICROSCOPE [CH V 



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. too) 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 Figs. 115- 



116. 



§ 179. 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 7'ight 



angles to the d?-awing sznface (Figs. 112, 114.) 



§ 180. 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 horizon- 

 tal, 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. 

 112), the fields partlj- 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 neces- 

 sary 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 {\ 182), to have the Wollaston 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 micro- 

 scope 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 (\ 162). 



§ 181. *Abbe Camera Lucida. — This consists of a cube of glass 

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



*For some persons the image and drawing surface, pencil, etc., do not appear 

 on the drawing board as stated above, but under the microscope, according to the 

 general principle that "objects appear in space where they could be touched 

 along a perpendicular to the retinal surface stimulated," — that is in the line of 

 rays entering the eye. This is always the case with the Wollaston camera lucida. 

 The explanation of the apparent location of the image, etc., on the drawing board 

 with the Abbe camera lucida is that the attention is concentrated upon the draw- 

 ing surface rather than upon the object under the microscope (Dr. W. B. 

 Pillsbury). 



