240 PEACTICAL PHOTO-MICROGRAPHY 



on these papers is produced by development, the result 

 also may be controlled by varying the constituents of the 

 developer, or by stopping the action at an earlier or later 

 stage. 



The printing process is carried out by placing a piece of the 

 paper, as before, in contact with the negative in a printing-frame, 

 and then exposing it to a suitable artificial source of light. 

 Almost any light-source may be utilised, except that it is 

 generally not advisable to have it of too high actinic value. 

 An ordinary gas-flame, an incandescent electric lamp, or an 

 oil lamp, may be used at will. 



To ensure fairly uniform results, some arrangement should 

 be made for making the exposures at definite distances from 

 the light-source. With ordinary illumination the light-action 

 will vary as the square of the distance ; so that if the distance of 

 the light-source from the printing-frame is doubled, the exposure 

 will require to be quadrupled to obtain the same result. In 

 practice, it is found that increasing the distance of the source 

 of light tends to decrease its penetrating power, hence a some- 

 what harder result is obtained by thus increasing the distance 

 even if the proper proportionate exposure is given. No definite 

 rule can be laid down as to exposure, for it varies with the 

 source of light, and the rapidity of the paper itself. Two 

 or three trial exposures, in conjunction with the instructions 

 given by the makers of the paper, will indicate the time to be 

 given. The results should be recorded so that under the same 

 conditions they may always be repeated. 



Development may be effected by almost any developer 

 except pyrogallic acid. Metol-hydrokinone, already mentioned 

 in dealing with negatives, may be utilised, and is in general 

 quite satisfactory. Possibly the best developer for bromide 

 work is amidol, as it gives a very pure black image, and is 

 entirely free from any tendency to stain. A 5 % stock solution 

 of sodium sulphite may be made up, and when it is required for 

 use, one grain of amidol per ounce of solution may be added. 

 Amidol does not keep well in solution, so that enough should 

 only be made up to be used at the time, or at the utmost, within 

 a day or two. The exact proportion of amidol is not of im- 

 portance, and it is generally sufficient, after having weighed out 

 the required quantity once or twice, to make an approximate 



