608 LIGHT AND ENERGY LOSSES IN PROJECTION [Cn. XIV 



radiation constitutes only about 10% of the energy radiated by the 

 arc, this effect is insignificant in comparison to the heating due to 

 the infra-red. Even when the water-cell is used only 43% of the 

 energy which gets through is visible as light. A greater thickness 

 of water would reduce this effect but little, hence it is necessary to 

 carry the heat away from the specimen as rapidly as possible. 

 This is done by the stage cooling cell which is in contact with the 

 glass slide. The effect is purely one of conduction, and a thick 

 piece of any transparent substance would answer. But water has 

 been chosen because of its great specific heat, and the comparative 

 cheapness of hollow glass cells. 



3,000 CP 



FIG. 342. ILLUSTRATION OF THE LIGHT AND ENERGY LOSSES IN THE 



PROJECTION MICROSCOPE. 



Starting from the arc lamp the light and energy reaching the first face'of 

 the condenser are each designated by 100%. Opposite each element of the 

 optical system is given the percentage of light and of energy transmitted by 

 each. With the 16 mm. objective only about 6% of the original light is avail- 

 able for the screen picture. 



853. Keeping the condenser cool. One of the causes of the 

 condenser breakage is the stream of hot gases from the arc which 

 strikes the upper part of the condenser and heats it unequally. 

 This is specially troublesome when the lantern is tipped up at an 

 angle. To prevent this a thin sheet of glass (watch glass) or mica 

 may be used between the arc and the condenser. Glass is to be 

 preferred as it is more transparent than mica and has less defects 

 to cause shadows on the screen. The following data refer to two 

 sheets of mica new and in good condition. 



Light absorbed Energy absorbed 



Thin piece 25.3% 27.5% 



Thick piece 36.0% ,35-% 



