SUNDRY APPARATUS 71 



used as any of the other forms. With this apparatus the 

 instrument is placed in a vertical position, and the drawing paper 



Fio. 80.- 



-No. 3371, Modified Abbe Camera 

 Lucida. 



Fig. 79.— No. 3370, Abbe Camera Lucida. 



placed on the table at one side. The mirror must be inclined 

 at such an angle that the centre of the field of view appears 

 below the centre of the mirror, or a distortion in the picture will 

 be caused. This generaUy limits the size of the drawing to a small 

 portion of the centre of the field of view, because of the closeness 

 of the mirror to the side of the microscope. This can be remedied 



if the paper on which 

 the drawing is to be 

 made be tilted up so 

 that the distortion is 

 corrected, for the image 

 can then be thrown to 

 a greater distance to 

 the side of the instru- 

 ment. In order to find 

 the correct angle at 

 which the paper should 

 be tilted to avoid distortion, the circular margin of the field of 

 view as seen upon the paper may be measured in two directions, 

 sideways and fore and aft, and the angle of the paper altered 

 till the two measurements are the same. This method can also 

 be adopted with the Beck vertical camera lucida, when it is 

 required to set the inclination of the microscope or drawing-board 

 to the correct angle experimentally. 

 A camera lucida (Fig. 80) of the 

 Abbe type is made in which the 

 bulky mirror is replaced by a small 

 tilting prism attached close to the 

 eyepiece, and the prism is halt- 

 silvered. In this case the drawing- 

 board must always be placed at an 

 angle which can be ascertained as ex- 

 plained above. A lens and neutral 

 tint glass can be used in the same manner as previously described. Drawing 



A table (Fig. 81) which can be set at any desired angle is '*'''*• 

 supplied which is a convenience where the drawing paper requires 



Fia. 81.— No. 3375, Draw- 

 ing Table. 



