3o8 



MAGNIFICATION OF OBJECTIVES AND OCULARS [Ch. IX 



§ 478. Magnification due to the ocular. — To find this experimen- 

 tally use a positive ocular with an ocular micrometer or a filar mi- 

 crometer, or remove the field-lens of the Huygenian ocular in whidi is 

 present an ocular micrometer. Make the tube-length i6o or 250 mm. 

 Focus the stage micrometer on the ocular micrometer and see what 

 the objective magnification is. For example, suppose the objective 

 real image of ^V nini- covers 2 mm. on the ocular micrometer, the 



Ocular 



Objective 



Fig. 183. Diagram Showing the Rays and Images when both an 

 Objective and an Ocular are Used. 



(From Optic Projection). 



Object The object of which an image is desired. 

 Objective The optical combination forming the first image. 

 Ocular Huygenian ocular projecting the screen image. 

 / 1 Field-lens diminishing the real image of the objective from r' V to r i. 

 r i Real image produced by the objective and the field-lens. 

 r' V This would be the size of the real image if no field-lens were used. 

 e I Eye-lens serving to project the real image (r i) to the screen. 

 Screen Image The real image on the screen projected by the eye-lens. 

 Used in the ordinary way with the eye next the ocular a virtual image is pro- 

 jected (fig. 78, 182). 



magnification in that case is 20. Now use the WoUaston camera 

 lucida and project the virtual image 250 mm. and get the magnifica- 

 tion of the entire microscope as directed in § 234. Suppose it is 200 

 diameters. It is known that the objective magnifies 20 diameters 

 and to get 200 diameters there must be a second or ocular magnifi- 

 cation of 200 -i- 20 = 10. That is, the ocular in this case magnifies 

 the objective real images 10 diameters, making the magnification of 

 the microscope as a whole 20 x 10 = 200. 



This was without the field-lens. Put the field-lens in place and get 

 the magnification of the entire microscope again. It will be markedly 



