230 PEACTICAL PHOTO-MICROGEAPHY 



when development has proceeded for a sufficient and proper 

 time : 



1. By direct observation of the plate during the development; 



2. By time ; 



8. By that known as the ' Factorial ' method. 



In the first case the developer is allowed to act until all the 

 detail that is required to appear in the resulting photograph 

 can be well seen on the plate, the method therefore being one 

 in which a certain amount of experience and careful observation 

 are essential. The time-method eliminates the personal factor 

 to a large extent, the plate simply being put in a given solution 

 at a certain temperature for a given length of time, and when 

 such time has elapsed the plate is removed from the developer, 

 washed, and at once fixed. 



In the * Factorial ' method the point to determine is the 

 time that elapses between putting the plate into the developer 

 and the first appearance of the image. This time is multi- 

 plied by a certain factor this latter depending on the developer 

 being used and the total time to be allowed is then the time 

 that the image takes to appear, multipb'ed by the development 

 factor. In this case again, therefore, the personal element does 

 not come in, the method being more or less mechanical. The 

 majority of workers, however, will in all probability at least at 

 first confine themselves to the first method in which the image 

 is directly observed to determine when development has pro- 

 ceeded far enough ; and for photo-micrographic work it is more 

 than probable that this is the most suitable method. It usually 

 happens that it is not so much a question of bringing out a 

 picture with perfect gradation, as of obtaining one in which 

 there is considerable differentiation between the parts of an 

 object, and for this purpose the observational method is 

 unquestionably the most satisfactory one. 



It must not be overlooked in all photographic operations, 

 that the temperature of the room in which the work is being 

 carried out, and of the solutions, is one affecting the result 

 very largely ; particularly does it affect the time that the various 

 operations take to perform. 



The developers that will be found useful for negative- 

 making are the following : 



Pyrogallic acid, hydrokinone, metol, amidol, ortol. 



