CH. V] DRA WING WITH THE MICROSCOPE 151 



(E) If a drawing of a given size is desired and it cannot be 

 obtained by any combination of oculars, objectives and lengths of 

 the tube of the microscope, the distance between the camera lucida 

 and the table may be increased or diminished until the image is of 

 the desired size. This distance is easily changed "by the use of a 

 book or a block, but more conveniently if one has a drawing board 

 with adjustable drawing surface like that shown in Fig. 130. 



(F) It is of the greatest advantage, as suggested by Heinsius 

 (Zeit. w. Mikr. , 1889, p. 367), to have the camera lucida hinged so 

 that the prism may be turned off the ocular for a moment's glance 

 at the preparation , and then returned in place without the necessity 

 of loosening screws and readjusting the camera. This form is now 

 made by several opticians, and a quadrant is added by some. (Fig. 

 128, 132.) Any skilled mechanic can add the quadrant. 



§ 207. Magnification of the Microscope and size of 

 Drawings with the Abbe Camera Lucida. — In determining the 

 standard distance of 250 millimeters at which to measure the image 

 in getting the magnification of the microscope, it is necessary to 

 measure from the point marked P on the prism (Fig. 124) to the 

 axis of the mirror and then vertically to the drawing board. 



In getting the scale to which a drawing is enlarged the best 

 way is to remove the preparation and put in its place a stage 

 micrometer, and to trace a few (5 or 10) of its lines upon one corner 

 of the drawing. The value of the spaces of the micrometer being 

 given, thus : 



,i T t!l mm. 



Fig. 131. Showing the method of indicating thescaleat luhich a drawing 

 was made. 



The enlargement of the figure can then be accurately deter- 

 mined at any time by measuring with a steel scale the length of the 

 image of the micrometer spaces and dividing it by their known size. 



Thus, suppose the 5 spaces of the scale of enlargement given 

 with a drawing were found to measure 25 millimeters and the spaces 

 on the micrometer were j^ millimeter, then the enlargement is 

 25-=- T f 7 =5oo. That is, the image was drawn at a magnification, 

 of 500 diameters. 



