230 



MICRO-PROJECTION WITH OCULARS [Cn. IX 



FIG. 127. HUYGENIAN OCULAR IN SECTION. 

 (From The Microscope). 



F. L. Field lens. This aids the objective in forming a real image. 



D Diaphragm in the ocular. It is at this level that the real image is 

 formed in ordinary microscopic observation. 



E. L. The eye lens. In projection this acts like an objective and projects 

 upon the screen an image of the real image (see fig. 207). 



A xis The optic axis of the microscope. 



E. P. Eye-point or Ramsden's circle. 



357. Projection oculars. Any ocular may be used for pro- 

 jection. The lower powers, x 2, X3, x 4, x 6, ( 357a) are 

 better than the higher powers, for they cut down the field less, there 

 is less loss of light, and there is not an inordinate magnification. 



Concave lenses having a virtual focus are indicated by the minus sign ( ) . 



If the dioptry of a lens is given, to find the principal focus: divide i meter 

 by the dioptry. For example, the dioptry of the amplifiers mentioned above 

 ( 356) is 5 for one and 10 for the other. Their foci are then i meter, 



~5 

 i meter. That is, they are concave lenses of 1/5 and i/io of a meter focus. 



-10 



On the other hand, to find the dioptry of a lens whose principal focus is known, 

 divide i meter by the principal focus and the result will represent the dioptry 

 of the given lens. Taking the same case as before where the amplifiers have 



principal foci of 1/5 and i/io meter, 



As the lenses are 



known to be concave, the minus sign is placed before the dioptry: 5, 10 

 diopters. 



The increase in magnification given by the amplifiers, 5, 10 was found 

 to average 1.7 for the 5 and 2.5 for the 10. The average was obtained by 

 considering all the screen distances and all the different objectives shown in the 

 table, 391. See also 392a. 



