SCEEENS PLATES EXPOSUKE 223 



decreasing transparency. It follows that if the sensitometer is 

 placed against the ground-glass focussing-screen of the camera, 

 the image being set up and illuminated, it will be possible to 

 read the numbers on the squares up to a certain point. A 

 number will be reached after which it becomes difficult to 

 decipher further. The number that can be read without 

 difficulty may then be taken to indicate relatively the value 

 of the light under those conditions. 



To obtain more exact indications from the sensitometer when 

 used for exposure purposes, it will be necessary first to make 

 some trial exposures. It should at once be said that it is 

 hardly a suitable method for use with high-power illuminants, 

 as with these the range of densities on the screen is not 

 sufficiently great, and it will probably happen that the densest 

 square of the sensitometer can be read with ease. This may 

 be partly overcome by interposing one or two thicknesses of 

 grey glass between the sensitometer and focussing-screen, so 

 that the power of the light is lessened by a definite amount ; the 

 same glass must of course be used on every occasion when it is 

 desired to estimate approximately the power of the transmitted 

 light. In practice it will be necessary to make a number of 

 trial exposures. Having determined the correct exposure for 

 any one number it is then easy to determine it for any other 

 visible number. As an approximation the following short 

 table may be regarded as an indication of the method as it 

 would apply in actual work with a source of light of moderate 

 power, such as acetylene : 



Last number visible . . 25 20 15 10 

 Exposure in seconds 1 4 16 64 



and so on, in regularly progressive increase. 



It must clearly be understood that the use of the sensito- 

 meter is but an approximation, and it must only be used, and 

 what it indicates must only be acted on, after due consideration 

 of all the other factors that require to be taken into account. 



It is obvious that the working N.A. of the objective and 

 of the sub-stage condenser would greatly influence the question 

 of exposure, as on these factors the light-transmitting power 

 of the optical combination depends. It has been suggested 

 that exposure might be determined solely in relation to this 



