1878.] On the Photometry of the Magneto-Electric Light. 157 



Edward A. Schafer, M.R.C.S. 

 Arthur Schuster, Ph.D., F.R.A.S. 

 Michael Scott, M. Inst. C.E. 

 Prof. Harry Govier Seeley, F.L.S. 

 Samuel Sharp, F.G.S., F.S.A. 

 John Spiller, F.O.S. 

 Hermann Sprengel, Ph.D. 



George James Symons, Sec, M.S. 

 Sir Henry Thompson, F.R.C.S. 

 Charles S. Tomes, M.A. 

 Edwin T. Truman, M.R.C.S. 

 Benjamin Williamson, M.A. 

 Prof. Edward Percival Wright, 

 M.A., M.D. 



The following Papers were read : — 



I. "On the Photometry of the Magneto-Electric Light." By 

 Captain W. de W. Abney, R.E., F.R.S. Received February 

 5, 1878. 



At intervals during the last three years it has been my duty to 

 ascertain the value of the illuminating power of different sources of 

 light, which have been brought before Government for military pur- 

 poses. As there is nothing secret in the results obtained, it seemed to 

 me that I might communicate them to the Royal Society, I have 

 obtained permission so to do. 



There are certain difficulties to be overcome which affect the 

 measurements, as it will be evident that the question of colour enters 

 largely into them, which are not so great when lights produced by 

 comparatively low temperatures are in question. 



My first attempt to obtain a comparison of the value of any two 

 lights was made by the method of extinction, that is, by introducing 

 between the light and the eye a screen graduated in opacity. After 

 comparing this method with that which I have recently adopted, I am 

 not inclined to quarrel with it. Originally I employed a wedge of 

 green-black glass for the screen, but owing to a complication in the 

 results due to different coefficients of absorption for different parts of 

 the spectrum, I had resort to strips of photographically graduated 

 glass, the necessary black colour being obtained by toning with a 

 platinum salt, and a consequent deposition of platinum black. With 

 a couple of these slips back to back, and with the ends showing the 

 greatest opacity placed together, a graduation was obtained varying 

 between nearly total transparency and an opacity sufficient to cut off the 

 direct rays of the strongest sunlight even when the eye was in its most 

 sensitive condition. These strips being cemented together, and the 

 relative opacity of every division of a small scale having been ascer- 

 tained, they were caused to pass in front of a narrow slit which was 

 open to the light to be tested. When all trace of light had dis- 

 appeared readings were taken, and from the series the value of the 

 light calculated. The great defect in this method is the variation in 

 the sensitiveness of the eye, and when employing it, it is necessary to 



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