222 PKACTICAL PHOTO-MICKOGKAPHY 



It was also shown that diffraction and scattering of the light 

 becomes less as the wave-length of the light used increases. It 

 therefore becomes even more necessary to use a suitable plate 

 when violet or ultra-violet light is being used as an illuminant. 



Exposure. Having set up the microscope and properly 

 adjusted the optical system, it is then necessary to determine 

 what exposure shall be given. It must be admitted that this 

 is not an easy matter, and that it is one the determination of 

 which with varying objects can only be arrived at as the 

 result of experience. No definite rules can be laid down, but 

 in general, exposure depends on three chief factors : 



1. The actinic value of the source of light ; 



2. The speed of the plate ; 



3. Magnification. 



Other factors may have to be taken into consideration in 

 determining the exposure particularly the character, colour, 

 and density of the object and also the colour-screen used. The 

 latter is, however, usually a known constant factor, so that the 

 necessary allowance can always be made for any given screen. 



All so-called methods of determining exposure are based 

 on the assumption that the exposure is correct when a sufficient 

 light-action has taken place in parts of the projected image 

 in which there is no light- obstruction. This assumes that 

 an uncoloured object is in the microscope which is delineated 

 entirely by perfectly transparent and perfectly opaque areas. 

 The exposure in such a case would, of course, be correct if 

 sufficient light-action had taken place in the transparent parts ; 

 but, unfortunately, the number of such objects is extremely 

 limited, and, in actual practice, the conditions are rarely exactly 

 complied with. 



A very simple appliance for estimating the relative in- 

 tensity of illumination at the position of the focussing-screen 

 is the Warnerke or Chapman-Jones sensitometer, the use of 

 which was first suggested by Dr. Bousfield. The appliance 

 as sold commercially is really intended for determining the 

 speed of sensitive plates, and consists of a glass plate divided 

 up into a number of squares which differ in transparency to 

 light. Each one is numbered and bears a definite relationship 

 in its light-transmitting power to the square preceding it; 

 the numbers run from one to twenty-five with regularly 



