314 Captain W. de W. Abney on the Photographic 



different parts of the spectrum the sensitiveness of the silver 

 compound is materially different, and that consequently the 

 absorption at different parts cannot be well compared. The 

 objection vanishes, however, at once, if ordinary precautions 

 are taken ; and as an illustration I will take a case. 



The absorption of a violet (cobalt) glass was required to be 

 registered photographically. A spectroscope having two 

 prisms of 62° was judged to give sufficient dispersion; and a 

 lens was used in the camera of a focal length of about 2 feet. 

 This gave a spectrum about 4 inches long, including the 

 visible and invisible radiations. The plate having been placed 

 in the camera, the top half of the slit was shielded, and sun- 

 light was reflected onto the bottom half for two minutes; the 

 sunlight was diverted, and the absorbing medium (in this case 

 violet glass) was placed in front of the slit*, the lower half 

 covered up, and sunlight again reflected onto the top half of 

 the slit for two minutes more. The plate was then developed, 

 and a print taken from the negative. A scale of shade having 

 been prepared, the following diagram was drawn from the 

 measurements made with it. 



The top continuous curve of fig. I. shows the intensity pro- 



f/c.i 



A D 



F 



Q / 



/ 







FIG. II 





\ 1 



Vhr. 



■Si ' 



1 





A 



II 



duced on the plate by unscreened sunlight. The dotted line 

 in the same figure shows the curve obtained when the cobalt 

 glass is interposed. 



I would here remark that care is necessary not to introduce 



* When this paper was communicated to the Physical Society, Prof. 

 Macleod suggested that the absorption of a liquid might be better demon- 

 strated if a wedge-shaped vessel containing it were placed in front of a 

 longer slit, of which a small image might be produced at the focus of the 

 collimating lens. This is quite practicable, as Professor Macleod and my- 

 self have found by actual experiment ; and if the image of coloured liquid 

 be corrected by a similar wedge of colourless liquid of nearly the same 

 specific gravity, there is no inconvenience attaching to it. 



