The Effective Temperature of tJie Sun. 315 



radiation from such an enclosure confirms in a remarkable manner 

 Stefan's law of radiation, viz., R = T 4 .* Since therefore the results 

 of several independent investigations corroborate this law, I have felt 

 justified in applying it to the results of my observations. 



On consideration it seemed that the most convenient form of 

 radiator would be a long tube closed at one end, and uniformly heated 

 in a gas furnace. Accordingly a porcelain tube, 2 feet in length and 

 1 inch internal diameter, was fitted into a Fletcher gas-tube furnace. 

 This was afterwards changed for an iron tube, which was employed in 

 the observations on September 30th, given below. 



A plug of asbestos was inserted in the tube at about 10 inches from 

 the end farthest from the radio-micrometer, and resting against this 

 plug was the end of a Callendar platinum-resistance thermometer. 

 This was connected with one of Professor Callendar's electric recorders, 

 so that during an experiment the temperature of the tube was 

 registered continuously on the paper wrapped round the drum of the 

 instrument. In front of the open end of the tube, and between it and 

 the radio-micrometer, was placed a large brass water-screen, through 

 which a copious supply of water passed. In front of the aperture (B) 

 of the radio-micrometer this screen was provided with a rectangular 

 aperture. One side of this aperture was formed by a slide moved by 

 a micrometer screw reading to O'Ol mm. By this means the area of 

 this aperture at any time could be measured with precision. Its fixed 

 sides were 5 mm. apart, and as the movable side had a range of 5 mm., 

 the maximum area of the aperture was 25 sq. mm. The distance (d) 

 of this aperture from the surface of the thermo couple was 66'3 mm. 



To make an observation the tube was heated to as high a temperature 

 as the furnace was capable of, and when a steady temperature had 

 been obtained, the amount of radiant heat coming from the interior 

 of the hot tube and passing into the aperture (B) of the radio-micro- 

 meter was adjusted by the micrometer screw until a balance was 

 obtained with the radiation coming from the Sun through the 

 aperture (A). 



If p is the angular semi-diameter of the sun, its radiation outside of 

 our atmosphere is K.TT sin 2 p, where K is a constant depending on the 

 sun's temperature. 



Again, if a be the height of the slit through which the radiation from 

 the hot tube reaches the radio-micrometer, and (3 its width, the 

 radiation may with sufficient accuracy be expressed by Kx(3/d' 2 . 

 Assuming Stefan's law, the radiation of the sun outside our atmo- 

 sphere is #% sin 2 p, 6 being the effective temperature of the sun. 



Stefan, 'Sitzber. d. k. Akad. zu Wien,' vol. 79, 1879 (Abth. 2), p. 391 ;; 

 Boltzmann, 'Wied. Ann.,' TO!. 22, 1884; M. Planck, 'Drude Ann.,' vol. 1, No. 1,. 

 1900 ; Pascben, ' Wied. Ann.,' vole. 58, 60, 189fi, 1897 ; Lummer and Pringsheim,. 

 ' Wied. Ann.,' vol. 63, 395, 1897. 



VOL. LXIX. Z 



