Gradation of a Developed Photographic Image. 305 



In percentages the transmission of white light through No. 1 and 

 No. 12 is therefore 100 and 0*477 respectively, which allows a suffi- 

 ciently wide range of intensity to be investigated. The above numbers 

 represent then the absorption of white light," and also that of the blue 

 light coming through a slit placed at 26 "8 of the scale of the spectrum. 

 To obtain the scale in powers of - 2 for the other rays they must be 

 multiplied by the factors given in Table I, 



The star can now be used for the purpose for which it was prepared. 



Experiments with Fixed Time of Exposure. 



With the colour-patch apparatus a patch of red light was thrown on 

 the star backed by a sensitive plate, which could be revolved round 

 their central point in a special dark slide, and exposure was made to 

 the patch with the plates rotating for the time it was judged necessary 

 to cause an impression of each intensity of light. The rotation was 

 deemed necessary in case the light coming through the thick part of 

 the prism was more absorbed than that coming through the thin part. 

 The plate was then removed from the slide, and a scale of gradation 

 impressed on a part which had been covered up during the previous 

 exposure. The source of light used for this scale was an amyl-acetate 

 lamp placed at 4 feet from the plate, and the time was doubled for 

 each successive exposure. On development there was an image of the 

 star, each space in different densities, and alongside a graduated scale 

 of densities with which the star densities could be compared. Other 

 plates were exposed to other rays of the spectrum, those selected being 

 at the scale numbers recorded in Table I. As each separate image of 

 the star could be compared with the scale of gradation given by the 

 amyl-acetate lamp they could be compared with one another. 



Spectrum Sensitiveness of Bromo-iodide of Silver. 



The first sensitive salt of silver with which experiments were made 

 was the bromide of silver, to which a small quantity of iodide of silver 

 had been added. A spectrum of the electric arc light was impressed 

 on the gelatine plates prepared with this salt, and the sensitiveness to 

 the various rays ascertained by the plan given in a previous paper.* 

 (To facilitate a comparison of the results given in this paper with 

 the curve of sensitiveness the latter is drawn on the prismatic scale as 

 given above.) 



* "The effect of the Spectrum on the Haloid Salts of Silver," Abney and 

 Edwards, * Koy. Soc. Proc.,' vol. 47. Eead December 12, 1889. 



