and Laboratory Methods. 1489 



bromide shall be used, this being regulated by the judgment and experience of 

 the operator. I have found it very convenient to use none at all in some cases, 

 and again in others to stop the developer back so that an image will not appear 

 in less than six or eight minutes. 



When working for extreme contrast, as in reproducing book illustrations, 

 mechanical diagrams, etc., a plate sensitized specially for this purpose is abso- 

 lutely necessary. Such a plate is manufactured by the G. Cramer Dry Plate Co. 

 of St. Louis, and may be obtained of any dealer in photo supplies. They are 

 rather slow in their action and must be given a longer exposure than ordinary. 



For development, the solution must be as strong as possible and well stopped 

 back. I usually make up the developing solution in 200 c. c. quantities, " enough 

 to conveniently fill a 5x7 tray," after the following formula : 



Ortol, - - - 75 c. c. stock solution. 



Soda, - - - 100 c. c. 



l^romide, 10 per cent, sol., 25 c. c. 



After exposure, the plates should be brushed off and immersed in the 

 developer, when the image will flash up rather quickly, and stand out in very 

 strong contrast with the black background ; but do not remove until the entire 

 plate is perfectly black, even when held up to the red light, then pass it into acid 

 fixing bath. The black background will be found to be perfectly opaque, even 

 if held up to the sun, while the lines will stand out perfectly transparent. 



Should any of my readers start using Ortol, I feel fully satisfied that it will 

 be some time before they change again. R. P. Woodford. 



Contributions to Our Knowledge of Color in Photo- 

 micrography.'^ 



One of the most perplexing and as yet unsolved problems of Photo-micro- 

 graphy is that of color values, i. e., how to reproduce natural colors by 

 means of the sensitive plate. Of the plates now in use, the orthochromatic ap- 

 proaches most nearly the ideal color plate ; yet this is not perfectly satisfactory, 

 as it does not give sufficient contrast. 



The investigations which form the basis of this article were undertaken to 

 determine the relative merit of various photographic plates. The apparatus, as 

 illustrated in Fig. 1, consists of a direct vision spectroscope so mounted in the 

 front board of an ordinary camera (with lenses removed) that the spectrum, 

 when projected on the plate, will come in the center horizontally and at the top 

 of the plate. The back of the camera is constructed in such manner as to allow 

 of its being moved in the vertical plane, thus making it possible for one to make 

 four exposures on the same plate, and by so doing to make an accurate com- 

 parison between them. 



The plates examined may be grouped according to their degree of perfection 

 as follows : 



Group I — Characterized by a very high degree of sensitiveness a little above 



* F. L. Richardson. Journal of the Boston Society of Medical Sciences, S : 460-464. 



