84 PHOTOMICROGRAPHY 



surface of about J inch in diameter would be very 

 suitable. 



No lamp gives a perfect illumination and the draw- 

 backs of a few of the most convenient must be considered, 

 it will then be easier to select the most suitable in any 

 given circumstances. The intensity of large radiants 

 is not even over the surface, and nearly all show structure 

 or inequalities of light when their image is focussed 

 across the object. The mesh of an incandescent mantle, 

 and inequalities in a flame, or /in the crater of an arc 

 lamp, are cases in point. 



The light must be absolutely steady, and, for con- 

 venience, it should always give the same illuminating 

 effect. The less heat given off by a radiant the better ; 

 the more intense illuminants suffer greatly in this respect, 

 and it is dangerous to use them with delicate preparations 

 or valuable objectives without a cooling trough. 



The spectrum of the radiant is a point of importance, 

 constant results cannot be obtained unless the com- 

 position of the light is invariable. Monochromatic light 

 is very beneficial in certain directions ; the results of 

 defects existing in the objective can be minimised, 

 and the photomicrograph improved by its employment 

 (Chapter IX). The ease with which it can be obtained 

 from any given light source must therefore not be 

 overlooked. 



SUNLIGHT. The sun would be the best of all illuminants 

 if its light could be relied on for use at any time, but even 

 in the most favourable localities some form of Heliostat 

 is necessary to project a steady beam into the microscope. 

 Heliostats are complicated and expensive instruments, 

 and, considered in conjunction wi'th the uncertainty of 

 sunlight, generally put the sun out of court as a source 

 of illumination for photomicrography. The variation 

 of the actinic rays of daylight at different times of the 

 day also introduces uncertainty in the exposure of the 



