ARC LAMPS 



85 



photographic plate, and, on that account, the relative 

 constancy of an artificial illuminant is a great point in 

 favour of always using one or other form in preference 

 to daylight. It is possible to dispense with a clockwork 

 heliostat and direct the beam of sunlight by a careful 

 manipulation of the mirror of the microscope, or, better 

 still, to set up a second reflector in such a position that 

 it can be made to follow the sun, and throw the light 

 on to the substage mirror. This simple device is scarcely 

 satisfactory, however, as it is not possible to control the 

 illumination very exactly thereby. 



North light, or sunlight reflected from white clouds, 

 is not suitable for critical work, and is not of sufficient 

 intensity for high powers, but is very convenient for 

 low-power work. This is particularly the case when a 

 vertical camera, which can be readily moved into a 

 good position, is used. 



ELECTRIC LIGHT. On account of the difficulties in- 

 volved in the use of the sun as an illuminant for photo- 

 micrography recourse must be had to artificial radiants, 

 and of these the electric arc stands pre-eminent. Under 

 the best conditions a perfectly steady light is obtained, 

 and, with direct current, the crater forms a radiant 

 surface that closely approximates to a small even disc. 



Arc Lamps. These are made for direct and alternating 

 supplies, but a perfectly steady light is never obtained 

 from an alternating current, and the arc generally moves 

 so markedly as to be useless for critical work. For many 

 purposes, however, when a brilliant but not critical light 

 is required, an alternating arc can be used, but very 

 great care is needed in the adjustment to prevent un- 

 even illumination, which may occur in the few moments 

 intervening between final adjustment and exposure, or 

 even in the middle of the exposure. 



Arc lamps are made to work with almost any~current 

 from 4 to 60 amperes or upward, giving a light that 



