602 LIGHT AND ENERGY LOSSES IN PROJECTION [Cn. XIV 



840. Losses by the removal of the margin of the beam. 



From the source, light spreads out in all directions. Only the light 

 that strikes the front surface of the first lens of the condenser is 

 available, hence the first lens should be of such a diameter and so 

 placed that it takes in as large an angle of light from the source as 

 possible. The use of a meniscus lens next the radiant allows a much 

 larger angle of light to be used than does a condenser without such 

 a lens (fig: 332, 821). 



The lenses after the first, should not remove any of the border 

 rays of the light transmitted by the first lens, or the first lens need 

 be of only sufficient diameter to furnish a beam of light which will 

 just fill the opening of the other lenses. 



After passing through the condenser the light is available for 

 illuminating the object. With the magic lantern the entire 

 diameter of the cone of light passes through the objective and 

 reaches the screen. With moving picture projection the entire 

 cone of light may or may not get through the objective ( 825, 829). 

 With the microscope, except for the lowest powers, the objective 

 lenses are smaller than the image of the crater which is thrown on 

 the front of the objective, and much loss of light occurs from this 

 cause. 



841. Losses by reflection. The polished surfaces of a lens 

 reflects some light, about 4 to 5 per cent, at each surface between 

 glass and air; 8 to 10 per cent, for each lens or plate of glass. If 

 the surfaces of the glass are not perfectly clean or perfectly polished 

 the light losses may amount to much more, sometimes 15% at each 

 surface. All reflected light being lost, this effect is generally much 

 more important than the slight absorption in the body of the glass 

 itself. A good illustration of this reflection by glass surfaces is the 

 brilliant reflection from windows often seen at sunset. 



842. Light losses by absorption. The object (slide, film, or 

 specimen) absorbs some of the light incident upon it. This is a 

 necessary accompaniment of showing the object at all, but an 

 object which does not absorb too much light is to be preferred 

 whenever obtainable. 



