no 



MAGNIFICATION AND MICROMETRY 



[CH. IV 



FIG. 100. Figure showing 

 the position of the microscope, 

 the camera lucida, the eye, and 

 the difference in size of the im- 

 age depending upon the dis- 

 tance at which it is projected 

 from the eye. (a) The size at 

 25 cm.; (b] at 35 cm., (\ 162). 



that the image could not be seen and measured at any other distance, but 

 because some standard must be selected, and this is the most common 

 one. The necessity for the adoption of some common standard will be 

 seen at a glance in Fig. 100, where is represented graphically the fact 



FIG. 101. Sectional view 

 of the Abbe Camera Lucida 

 to show that in measuring 

 the standard distance of 250 

 millimeters, one must meas- 

 ure along the axis from the 

 point P, at the left of the 

 prism, to the mirror, and 

 from the mirror to the 

 drawing surface. For a 

 full explanation of this 

 camera lucida, see next 

 chapter. 



FIG. 101. 



that the size of the virtual image depends directly on the distance at 

 which it is projected, and this size is directly proportional to the ver- 

 tical distance from the apex of the triangle, of which it forms a base. 

 The distance of 250 millimeters has been chosen on the supposition 

 that it is the distance of most distinct vision for the normal human eye. 



