216 Mr. Dyke on the Flux of Light from the 



mean ordinate or corrected ratio comes out 077 instead of 

 0*785 as it should do if the cosine law had been strictly 

 fulfilled. In this case the dotted line would have fallen on 

 the semicircle and coincided with it. 



Another similar set of measurements was made with a 

 straight oxide filament taken from a Kernst lamp used without 

 heating-coil, and with the supports of the filament all carefully 

 painted dead-black to avoid errors due to reflected light. In 

 this last case no correction due to loss of light by reflexion 

 from a glass envelope was needed, as the ISTernst filament 

 was used in the open air. The ratio of the mean spherical to 

 mean horizontal filament in this last case was experimentally 

 found to be 0*785, or exactly the value it should have if the 

 cosine law is obeyed. 



We have therefore in these experiments a justification for 

 the assumption made in this paper for the law of luminous 

 radiation from a straight incandescent filament. 



My thanks are due to Mr. Dyke for his assistance in the 

 experimental portion of this work. 



XXVII. On the Flux of Light from the Electric Arc with 



Varying Power- Supply. By G. B. Dyke, B.Sc* 



[Plate II.] 



IN 1896 a paper was communicated to the Physical Society 

 by Dr. J. A. Fleming, F.B.S., and Mr. J. E. Petavel t, 

 recording the results of numerous observations on the Electric 

 Arc, dealing, amongst other matters, with the question of the 

 relation between the flux of light, or mean spherical candle- 

 power, and the watts expended in the arc. 



The time and labour necessary to obtain the mean spherical 

 candle-power of any source of light from observations made 

 on the point-by- point method then employed, prevented any 

 very extended study of this relation. 



The construction of the integrating photometer, described 

 by the author in his paper " On the Practical Determination 

 of the Mean Spherical Candle Power of Incandescent and 

 Arc Lamps " J, having, to a very large extent, removed these 

 difficulties, it was thought that a more extended series of 

 observations in this direction might be undertaken, and 

 facilities for this purpose were kindly granted in the Pender 



* Communicated by the Physical Society : read March 24, 1905. 



f "An Analytical Study of the Alternating-Current Arc," bv J. A. 

 Fleming, M.A, D.Sc, F.R.S., and G. E. Petavel, Phil. Mag. April 1896, 

 p. 315. 



J " On the Practical Determination of the Mean Spherical Candle- 

 Power of Incandescent and Arc Lamps," by G. B. Dyke, B.Sc, Phil. 

 Mag. Jan. 1905, p. 136, and Proc. Phys. Soc. vol. xix. 



