CHAPTER XXI. 

 ENLARGING. 



THIS is a somewhat important subject to the photo-micro- 

 rapher, as it is frequently inconvenient to take at the first a 

 negative as large as may ultimately be required. " Enlarging," 

 as the word is technically used by photographers, will not, as 

 some persons seem to think, help us to get any superior quali- 

 ties to those which we can get by direct amplification properly 

 managed, excepting only the one quality of size. If we have 

 to produce a photograph of an uneven diatom exempli gratia, 

 at a magnification of 300 diameters, we shall get it just as 

 well, or better, by direct operation in the camera as by magni- 

 fying to 150 diameters in the camera, and then "enlarging" 

 to 300 diameters, always provided that in our original direct 

 operation we do not overtax our instruments. By "enlarge 

 ment" we enlarge and accentuate the difference of focal 

 planes in the original object just as much as we accentuate it 

 by direct projection ; and, what is more, we often introduce 

 new aberrations in our system of " enlargement," unless we 

 are tolerably aufait in our optics and careful in our operations. 



In considering enlargement we have two optical systems to 

 attend to. 1st. A system for collecting light and transmitting 

 it through our original negative or positive, or for concentrat- 

 ing the light passing through our original at or near a certain 

 point with reference to our projecting system. 2d. Our pro- 

 jecting system, which regulates the sizes of our enlarged image 

 and projects it upon our sensitive surface. 



Our condensing or transmitting system depends chiefly upon 

 the sensitiveness of our photographic receiving surface ; if the 

 latter is very sensitive and our light reasonably actinic, we 

 require no condenser at all. But if either our receiving surface 



