312 Dr. W. E. Wilson. 



;i well-known product of decomposition of chlorophyll. Another is a 

 substance of well-marked properties, nearly resembling, but not identi- 

 cal with, phyllocyanin. It has not, so far as my experience goes, been 

 hitherto observed as a result of any process of decomposition to which 

 chlorophyll has been subjected outside the animal body. I consider it 

 as a body $ui generis, characterised by its fine purplish-blue colour and 

 its brilliant metallic lustre. The existence of other products in addi- 

 tion to these two is possible. On one occasion, indeed, a definite 

 crystalline substance was obtained, which seemed to be peculiar, but 

 that it was in any way connected with chlorophyll could not with 

 certainty be maintained. 



"The Effective Temperature of the Sun." By W. E. WILSON, 

 D.Sc.,F.E.S. Eeceived December 5, Read December 12, 1901. 



In March, 1894, Dr. G. Johnstone Stoney communicated to the 

 Society a memoir by myself and Mr. P. L. Gray, entitled " Experi- 

 mental Investigations on the Effective Temperature of the Sun," which 

 was published in the Phil. Trans.,' A, vol. 185 (1894). In these 

 investigations the method we adopted was as follows : A beam of 

 sunlight was sent horizontally into the laboratory by means of a 

 Stoney single-mirror heliostat. The mirror was an optical plane 

 of unsilvered glass, and the beam was directed into one aperture (A) 

 of a differential Boys' radio-micrometer. The other aperture (B) 

 received the radiation from a strip of platinum, which could be 

 raised to any desired temperature by an electric current supplied by a 

 battery of accumulators. The temperature of this strip was at any 

 moment determined by its linear expansion, the instrument being 

 previously calibrated by melting on it minute fragments of AgCl 

 and of pure gold, as in Joly's meldometer. In front of the aperture (B) 

 of the radio-micrometer was placed a stop with a circular hole of 

 5 '57 mm., and the distance of this hole from the receiving surface 

 of the thermo-couple was 60 "2 mm. This gave for the angle subtended 

 by a diameter of the aperture at the receiving surface 5*301. Knowing 

 then (i) the ratio which the angular diameter of this circular aperture 

 bears to that of the sun, (ii) the temperature of the platinum strip 

 at the moment that the radio-micrometer is balanced, (iii) the amount 

 of the sun's radiation lost by reflection from the heliostat mirror and 

 also by absorption in the earth's atmosphere, it is possible on any 

 assumption with regard to the law connecting radiation with tem- 

 perature, to determine the effective temperature of the sun. A series 

 of very accordant observations were made in this way, the mean of 

 which gave 6200 C. as the effective solar temperature. 



