93 



MAGNIFICATION AND MICROMETRY. 



[CH. IV. 



Fig. 95. Figure showing the 

 position of the microscope, the 

 camera lucida, and the eye, and 

 the different sizes of the image 

 depending upon the distance at 

 which it is projected from the 

 eye. (a ) The size at 25 cm. ; 

 (b) at 35 cm., (§ 153). 



the simple and the compound microscope the directions were to measure 

 the virtual image at a distance of 250 millimeters. This is not that the 

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



^ 



Fig. 96. Sectional vit w 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, (Figs. 102,106). 



Fig. 96. 



cause 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. 95, where is represented graphically the fact 

 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 verti- 

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



