TH,E 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FOURTH SERIES.] 



SEPTEMBER 1874. 



XXVI. On the Opacity of the Developed Photographic Image. 

 By Captain Abney, R.E. } F.R.A.S., F.C.S.* 



IN a series of pictures of the sun which have lately been taken 

 by photography, I found the opacity of the image by no 

 means varied directly as the time of exposure. This caused me 

 to institute an inquiry into the relation of time of exposure and 

 intensity of light on the one hand, and the resulting opacity of 

 the image on the other. 



Primarily it was necessary to obtain some known gradation 

 of intensity of light, and then to measure the resulting opacities 

 caused by it on a photographic plate. The gradation was ob- 

 tained by causing a " star " to revolve rapidly round its centre. 

 The " star " was cut out with great exactness from white card- 

 board and made with eight " points." The curve of each point was 

 made to take the form of a portion of an equiangular spiral. 

 By this means an arithmetical progression of white was obtained 

 when the star was made to rotate. When revolving in front 

 of a black background, at two inches from the centre of the card 

 (and within that distance) pure white was obtained ; whilst at 

 fourteen from the centre pure black was obtained. The black 

 background employed was of such a dead nature that sunlight 

 gave no appreciable shadow on it when an opaque body was 

 placed before it. 



The star was made to revolve at the rate of fifty revolutions a 

 second. In some cases a dead-black star was made to rotate 

 before a clear sky, the only access of light being through the 

 openings of the points. 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 48. No. 317. Sept. 1874. M 



