246 PROCEEDINGS OF THE AMERICAN ACADEMY 



only about nine per cent of daylight under similar circumstances. Its 

 effect is therefore similar to yellow glass. The fact that the yellow- 

 violet combination was so satisfactory with the gas-flame was probably 

 due to the capacity of the yellow to cut off the small amount of violet 

 emitted by this source, while it was quite inadequate to destroy it in 

 the case of daylight, as is indicated by the figures. The precise 

 order here given is not insisted on, but only the general sequence, as 

 in the case of most of the media there was very little difference in their 

 desirability, and sometimes the position of two media on the list would 

 be reversed by a negative. One fact was brought out, however, with 

 great distinctness in all the series, and that was the great inferiority 

 of the yellow-green combination. This was of course to be expected, 

 but, as many photographers still use this medium in their dark rooms, 

 it was considered desirable to include it in the list, merely to show its 

 inferiority. Although in the second series Canary paper double 

 seemed to give the least fog of any of the media, the light transmitted 

 by it proved to be so faint that it was quite inadequate for satisfactory 

 illumination of the dark room ; and, owing to its opacity, some doubt 

 was cast on the accuracy of the observation. The second medium on 

 the list has therefore been selected as the best practical one to employ 

 with gas-light illumination. Its color is very pleasant to the eyes, and 

 being translucent, instead of transparent, it lights the whole room in a 

 very satisfactory manner. It is advisable to place it behind a sheet 

 of glass, in order to protect it from being soiled by the chemicals 

 employed. If one is obliged to use daylight as a source of light, it 

 is probable that a sheet of red glass in connection with one or two 

 thicknesses of Golden fabric would form as satisfactory a combination 

 as any. 



Experiments were next made to determine how bright a light is 

 permissible with the medium adopted. The fish-tail burner which 

 was used on the gas-lamp was placed fourteen inches behind a double 

 sheet of Golden fabric, measuring twenty-four by sixteen inches. An 

 extremely sensitive gelatine plate was placed at a distance of twelve 

 inches in front of the medium, and a portion of it exposed for one 

 minute. It was found, that if the gas-flame measured only one inch 

 in height, not the least trace of fog was produced by the exposure. 

 This is too faint a light for satisfactory use, however, and as a very 

 minute amount of fog does not injure a negative perceptibly, and the 

 direct exposure to the light in actual practice is much less than a 

 minute, the flame is turned up to a height of an inch and a half or two 

 inches for ordinary development. By this light one can readily see to 



