1894.] 



The Thermal Radiation from Sun Spots. 



247 



of the radio-micrometer, and the driving clock will then keep it 

 there. 



The observations are taken in the following manner. A small 

 screen is placed over the aperture of the radio-micrometer, and the 

 zero position of the spot of light on the scale noted. The screen is 

 then removed, and the umbra of a sun spot placed on the aperture. 

 The reading is then taken and entered in column u. The image is 

 then moved, so that a part in the neighbourhood of the spot, 



jut at the same distance from the centre of the solar disc, is placed on 

 the aperture. This reading is entered in column N. Finally, a 

 ling is taken at the centre of the disc, and entered in column C. 

 The throws of the instrument are then got by subtracting the figures 

 columns u, N", and C from the zero. The deflections of the instru- 



lent have been experimentally proved to be strictly proportional to 



ie amount of radiant heat falling on the thermo-couple. The 

 following is a typical observation taken August 7, 1893, of a large 

 sun spot then visible. The umbra of this spot measured 0'8 in. 

 icross on the screen, so that the aperture of the radio-micrometer 



pas only covering about -^Q of the apparent area of the umbra. 



The ratio 



umbra of spot 



.Q1 



= *"* = 0-292. 

 4-49 



neighbouring photosphere 

 Five concordant readings gave a mean deflection of 4*57 for the 



jntre of the sun, which gives for the ratio = 0*287. 



centre 



This spot was at a distance from the centre of the disc of about 

 0'4 of the radius. 



As the radiation from the photosphere falls off from the centre to 

 the edge of the disc, it seemed an interesting point to determine if 

 any change in the ratio of u/C would take place as a spot was 

 carried across the disc by the sun's rotation. If the spot' is, as is 

 generally thought, a depression, the absorption of heat ought to 

 icrease as it is carried towards the limb, on account of the increased 



