226 PKACTICAL PHOTO-MICKOGEAPHY 



actinometer must be exposed to the light in exactly the same 

 position that the image will occupy on the screen, the ground- 

 glass screen in this case being removed so that the light is 

 projected directly on to the exposure-meter. In the watch- 

 form of meter, such as Wynne's, there are two standard tints, 

 to either of which the sensitive paper may be allowed to darken 

 until it matches. The lighter one is the more convenient to use 

 with artificial light as the time occupied is shorter, being in 

 fact one quarter of that necessary to acquire the darker tint. 

 The exposure necessary will vary directly as the time indicated 

 by the meter ; so that one or two correct exposures having been 

 made experimentally, a record may be kept and future exposures 

 varied in accordance with the meter indications. It will 

 nearly always happen that one source of light will be adopted, 

 and variation from this will rarely occur. Should, however, 

 a change of illuminant become advisable for any reason, then 

 the following may be an indication of the relative actinic 

 value of some well-known illuminant s. Taking an efficient 

 type of oil lamp as a standard that is, one which has a round 

 wick and gives a fairly white flame its power in relation to the 

 electric arc is roughly represented as follows : 



An arc lamp taking 7 amperes equals approximately 39 oil lamps. 

 " > M 7o ,, ,, 



15 117 



20 160 



It will, therefore, be seen that the electric arc increases in 

 actinic efficiency as the current rises, which is entirely in 

 accordance with its increase of visual luminosity. 



An acetylene lamp, in relation to an oil lamp, is area for 

 area of flame of about five times its intrinsic brilliancy. Lime- 

 light, using a blow- through jet, would be from seventeen to 

 twenty times as actinic as the oil lamp ; and, using a mixed jet, 

 from twenty-seven to thirty-five times. The comparisons in 

 these cases are area for area of flame, and do not of necessity 

 represent the relative total illuminating power. The Nernst 

 electric lamp varies according to voltage ; but a 100-volt lamp 

 maybe estimated at approximately ten times the actinic value of 

 an oil lamp. With higher voltages the Nernst lamp increases 

 somewhat in efficiency, so that at 240 volts the increase 

 will approximate to thirty times. These estimations are but 



