604 LIGHT AND ENERGY LOSSES IN PROJECTION [Cn. XIV 



846. The energy losses. The energy losses occur principally 

 in three places (fig. 342). 



1. In the condenser lenses. 



2. In the water-cell, if there is one. 



3 . In the specimen. 



Energy losses beyond the specimen are not considered separate 

 from light losses. 



847. Losses in the condenser. The glass of which the con- 

 densers are made, even if perfectly transparent to visible light, 

 absorbs a large amount of infra-red. A piece of condenser glass 

 2 cm. thick was found to absorb 41% of the radiant energy from 

 the positive crater of the right-angle arc incident upon it, while 

 absorbing but 10% of the incident light. This has two effects. 



1 . The absorbed energy heats the condenser very greatly. 



2. The light which gets through the condenser has a much less 

 heating effect on the specimen than it would have otherwise. 



The first effect (heating the condenser) is a distinct disadvantage 

 to the condenser as it is one of the causes of condenser breakage. 

 Most of the energy absorbed will be by the first lens, and in that 

 one, more will be absorbed at the surface near the lamp than away 

 from it ; a circumstance which leads to unequal heating and puts a 

 strain on the lens. 



Different kinds of glass, equally transparent to visible light 

 absorb different amounts of infra-red. For example, crown glass 

 will be found to be opaque to some of the longer waves to which 

 flint glass is perfectly transparent. 



The second effect of heating the condenser is an advantage, as 

 the specimen is relieved of a good deal of the heating effect. Lan- 

 tern slides are less likely to be cracked, moving picture films are 

 less likely to curl or catch fire, and microscopic specimens can be 

 shown for a longer time before they are injured. 



848. Energy losses in the water-cell. Water is very opaque 

 to radiation of great wave length, even the thinnest films being 

 absolutely opaque to certain wave-lengths. 



The table ( 849) and the curves (fig. 340-341) show the energy 

 of the positive crater of the right-angle arc transmitted by layers 



