NEGATIVE MAKING 57 



screen upon the gelatine surface of its oppo- 

 site side and securely fixed in that position. 

 These lenses may be procured of various 

 forms and prices, the cheapest being the well- 

 known three-legged microscope costing fifty 

 cents. 



Prior to completing the final focusing, you 

 will notice that I have placed a carrier holding 

 a color screen of yellowish green between the 

 radiant and stage, in such manner as to pre- 

 vent any light other than that passing through 

 it from reaching the object. I then close the 

 iris diaphragm sufficiently to exclude the flood 

 of light entering by its full aperture in vol- 

 ume sufficient to drown much of the finest 

 details of the object. It is a very nice point 

 to determine the precise amount of illumina* 

 tion necessary for a given subject; to avoid 

 insufficiency on the one hand and harmful ex- 

 cess on the other. Practice and experience 

 alone can teach this matter of almost prime 

 importance. And observe carefully when using 

 a color screen that the focusing must be done 

 through it. It will not answer to place it in 

 position after the focusing has been com- 

 pleted. 



Having at last carried my readers through 

 the preliminary processes of making a nega- 



