142 



MAGNIFICATION OF THE MICROSCOPE 



[Ch. V 



focus at the standard distance of 250 mm., as shown in fig. 77, but 

 will remain parallel. At that distance then the image on the retina 

 would be a diffusion circle. In order that there be the appearance 

 of a point focus the distance must be great enough so that the parallel 

 rays from a point will be separated less than one minute (§ 226-227). 



Fig. 88-89. Diagrams of Simple and Compound Microscopes with 

 Parallel Beams Emerging above and Projected below. 



Axis The principal optic axis. 



Object The object. 



Objective The objective of the compound microscope. 



r i The real image formed by the objective. , 



Ocular-Magnifier The ocular and magnifier for the real image in the com- 

 pound microscope, and for the object in the simple microscope. 



Eye-point The most favorable position for the eye of the observer. 



Below, at 250 mm., the usual position of the projected image, no image is 

 formed with parallel rays. These only seem to come from a point at a dis- 

 tance where their separation is less than one minute (§ 226-227). 



Table of magnification and of the valuations of the ocular microm- 

 eter. — The table should be filled out by each student. In using 

 it for Micrometry and Drawing it is necessary to keep clearly in mind 

 the exact conditions under which the determinations were made, and 

 also the ways in which variations in magnification and the valuation 

 of the ocular micrometer may be produced. 



