244: PROCEEDINGS OF THE AMERICAN ACADEMY 



Investigations on Light and Heat, made and published tvholiy oe in part with 

 Appropkiaiion from the Rumford Fund. 



XL 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XXII. — COLORED MEDIA FOR THE PHOTOGRAPHIC 

 DARK ROOM. 



By William H. Pickering. 



Communicated May 13, 1885. 



Since the advent of the gelatine dry plate, many photographers have 

 complained that their eyes are injured by the dark red light generally 

 used in development. Considerable discussion of this subject has 

 been aroused of late, but the only experiments made, as far as can be 

 learned, have been to determine how much plates fog when exposed 

 at the same distance to the various media tried. The experimenter 

 then examines some object at that distance, and if he thinks he can 

 see it about as well, and the plate is less fogged, the medium is pro- 

 nounced an improvement. 



In order to obtain more satisflictory results the following problem 

 was proposed: — For a given distinctness of vision, what medium, or 

 combination of media, will give the least fog on a gelatine plate ? In 

 order to determine at what distance from the lights equal distinctness 

 of vision was obtained, a negative having some fine markings upon it 

 was held at such a distance from the various sources that the details 

 were just distinguishable from one another. Different portions of a 

 sensitive plate were then exposed at these distances, one after another, 

 to light coming through the different media, and the plate was then 

 developed. Exposures with both daylight and gas-light as sources 

 were made on the same plate, and, as was to be expected, those made 

 by daylight were much the most fogged. If more colored glass was 

 used, the daylight was too fxint. So, for this reason alone, gas-light 

 would be preferred. Add to this, that, if the gas-flame be placed at 

 the bottom of a properly constructed flue, it will serve to ventilate as 



