COOLING TKOUGHS 



149 



the rays in a parallel beam on to the substage condenser. 

 The lantern condenser is partially corrected. 



The adjustments are substantially the same as are 

 required when the radiant, instead of its image, is at the 

 focus of the paralleliser. 



The Cooling Trough. Whenever rays from an intense 

 source of light are concentrated on to the microscope, the 



Fig. 47 



ILLUMINATION BY A PARALLEL BEAM OF LIGHT 



necessity arises for precautions to avoid damage to lenses 

 and preparation by the heat of the radiant, and a 

 water-trough must be inserted as indicated in Fig. 47. 

 Water is the only liquid required, but the trough must be of 

 ample thickness : 3" or 4" is none too much for a 30-ampere 

 arc lamp ; a 5-ampere lamp will require a thickness of 

 about I". With less intense light sources there is no neces- 



