u8 DRAWING WITH THE MICROSCOPE. [CH.K 



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 by some. Any skilled mechanic 

 can add the quadrant. 



§ 177. Magnification of the Microscope and Size of Drawings 

 with the Abbe Camera Lucida. — In determining the standard dis- 

 tance 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. 102; 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, 



iw th mm - 



Fig. 112. Showing the method of indicating the scale at which a drawing ivas 

 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 width. 



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 TTroth millimeter, then the enlargement would be 

 2 5 ~=~ Tiro = 5°°- That is, the image was drawn at a magnification of 

 500 diameters. 



If the micrometer scale is used with every drawing, there is no need 

 of troubling one's self about the exact distance at which the drawing 

 is made, convenience may settle that, as the special magnification in 

 each case may be determined from the scale accompanying the picture. 

 It should be remembered, however, that the conditions when the scale 

 is drawn must be exactly as when the drawing was made. 



§ 178. Drawing at Slight Magnification. — Some objects are of 

 considerable size and for drawings should be enlarged but a few diame- 

 ters, — 5 to 20. By using sufficiently low objectives and different ocu- 

 lars a great range may be obtained. Frequently, however, the range 

 must be still further increased. For a moderate increase in size the 

 drawing surface may be put farther off, or, as one more commonly 



