DRAWINGS ON PHOTOGRAPHIC PRINTS. 



Salted silver-paper also requires a rather long printing with sunlight. When 

 properly printed the paper is washed in a bath of salt-water, rinsed in several 

 changes of pure water, and fixed in a weak solution of hypo (i : 15). 



Bromide-prints under the negative are usually made by exposing the paper 

 to artificial light at a standard distance, say 9 inches or 12 inches. By always 

 exposing at a given distance and to a light of uniform intensity two variable factors 

 are excluded, and one then has to take into account only the quality of the paper 

 and the density of the negative. It is usually economical, especially for beginners, 

 to test the density of the negative in advance by exposing, for various periods, 

 narrow strips of the sensitive paper laid across the negative so as to include dense 

 and thin portions. These strips are then developed, and if none of them have been 

 properly exposed, a second trial is made. When the right exposure has been 

 learned, the print is made. A little experience enables one to judge quite correctly 

 as to the proper exposure by simply looking through a negative. No very definite 

 rules can be given for length of exposure ; this depends so much on distance from 

 the light and brightness of the flame. With velox paper and an ordinary flat gas- 

 flame at a distance of 9 inches the writer's negatives usually require from 15 seconds 

 to 2 minutes. With a Welsbach light or with thin negatives the time would be 

 shorter. At a distance of 18 inches from the light the time would, of course, be 

 quadrupled. Directions for the employment of special developers usually accom- 

 pany each maker's paper. The writer has found ortol (p. 141) a very good developer 

 for velox paper, and prefers it to metol-hydro. Velox-prints are developed in weak 

 artificial light (gas turned low) ; they are rinsed from the developing solution by 

 passing quickly through a bath of pure water ; they are then fixed in hypo, and 

 washed for at least one-half hour in running water. The writer pins the prints to 

 a smooth board and floats these in a bath-tub or clean sink, film-side down. Most 

 of the curling may be avoided by drying the prints, film-side down, on mosquito- 

 netting stretched on a wooden-frame. 



The yellowing of prints is often due to the fact that they were not properly 

 fixed; the hypo solution was weak, or the time of exposure to it was not sufficient. 



All pen-and-ink drawings on such photographic prints must be made with 

 waterproof India ink, after which the photographic part is bleached out by exposure 

 for a few minutes in water containing cyanide of potash (i 1500, more or less). The 

 drawings should be exposed in this bath only as long as necessary. If any part of 

 the print refuses to bleach, it should be moistened with iodine potassium iodide and 

 returned to the cyanide bath. It is then passed through pure water and dried face 

 up on blotting paper in a place free from dust. 



SOME MILESTONES IN THE PROGRESS OF BACTERIOLOGY. 



The development of bacteriology can not be separated from advances in human 

 and animal pathology. Physicians and surgeons have made most of the brilliant 

 discoveries or have led the way to them. Chemists and physicists have assisted. 

 With a few shining exceptions, botanists have had comparatively little to do with 

 the advancement of this science. Bacteriological inquiry has been an incentive to 



