CH. V~\ DRAWING WITH THE MICROSCOPE 131 



the different structures could be shown, and by omitting some of the 

 fibers the others could be made plainer without an undesirable enlarge- 

 ment of the entire figure. 



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

 tained 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 adjust- 

 able drawing surface like that shown in Fig. 116. The image of a few 

 spaces of the micrometer will give scale of enlargement, or the power 

 ma}- be determined for the special case (§ 186-187). 



(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 prepa- 

 ration, and then returned in place without the necessity of loosening 

 screws and readjusting the camera. This form is now made by several 

 opticians, and the quadrant is added hy some. (Fig. 114.) Any skilled 

 mechanic can add the quadrant. 



§ 186. Magnification of the Microscope and size of Draw- 

 ings 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. 109) to the axis of the mirror 

 and then vertically to the drawing board. 



In getting the scale to wdiich 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 : 



,fotli mm. 



Fig. 117. Showing the method of indicating the scale at which a drawing 

 was made. 



The enlargement of the figure can then be accurately determined 

 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 



