CH. XIV] LIGHT AND ENERGY LOSSES IN PROJECTION 601 



This image is inverted with respect to D, but erect with respect to 

 the original object (fig. 207). A diaphragm at D, limits the size 

 of the field and makes its boundaries sharp. Often owing to the 

 small size of the diaphragm D, the field is not as large as desirable 

 on the screen. 



Besides limiting the size of the field there is a greater loss of light 

 with the ocular than with the amplifier as the ocular is made of at 

 least two separated lenses while the amplifier consists of but one 

 lens or a cemented combination. 



F IG - 339- IMAGE FORMATION WITH A PROJECTION OCULAR. 



O Objective forming a real inverted image D, with the help of the field lens 

 of the ocular 0,. 



0. Field lens of the ocular. 



O 2 Eye or projection lens of the ocular. It projects a screen image /, of 

 the real image D. 



The image D, wa? inverted by the objective. 2 , also inverts ths image D,in 

 projecting it, hence the final image / is erect like the object. (See also fig. 207). 



APERTURE AND LIGHT LOSSES 



839. So far the path of the light from the source to the screen 

 has been considered mainly from the standpoint of image formation, 

 no account having been taken of the amount of light needed or of 

 the losses of light and energy in the apparatus. 



Light losses may occur from three causes: 



1. Removal of the margin of a beam of light due to lenses of 

 insufficient diameter. 



2 . Reflection of light both regular and diffused, at the surfaces 

 of the lenses. 



3. Absorption of light by the glass of the lenses, by the partial 

 opacity of the object and by dirt. 



4. Special light losses due to the nature of the experiment. 



