674 HEPORT— 1903. 



tion absorbed is similarly kKH, where k is the coefficient of absorption which is 

 equal to the coefficient of emission. I shall also write 



<^/^*',*^/'^' 



We may then express some of the results obtained as follows, the complete investi- 

 gation being reserved for publication in the 'As trophy sieal Journal.' 



1. A plate of infinite thickness sends out an amount of radiation 



7 



where R is the radiation of a blaclt body having the temperature of the plate. 

 Thus if kIs = A, 1, or 2, (y - l)/y = -73, -83, or -92 respectively. 



A great thickness of a foggy vapour therefore does not tend to give a continuous 

 spectrum, but one of bright lines. The brightest line will be that which has the 

 greatest emissive power. 



2. An absorbing and radiating layer of a foggy vapour placed in front of a 

 luminous surface of higher temperature may show bright lines as well as dark 

 lines. 



3. The continuous spectrum transmitted through such a layer, if there is no 

 absorption, has an intensity 



2 + St 



where t is the thickness of the layer, and A the intensity of the incident light. A 

 line will be dark or bright according as the intensity belonging to its radiation 

 value smaller or greater than this. 



4. The radiation of the background and the coefficient of scattering being equal, 

 the brightest lines belong to the radiations of greatest emissive power. This 

 explains the absence of the helium line D., from the spectrum of the sun. 



5. Under the conditions probably holding in the stars, where in consequence 

 of lower temperature the ratio of black radiation of the absorbing layer to that of 

 the photosphere is decidedly greater for the red than for the violet radiations, the 

 less refrangible rays are more easily reversed than the more refrangible rays. This 

 probably accounts for the fact that stars are apt to show the less refrangible 

 hydrogen lines bright, and the more refrangible hydrogen lines dark. 



6. If the scattering is due to email particles, .so that the short wave-lengths are 

 much more scattered than the longer waves, the above result may be reversed, and 

 the most refrangible lines may be those most easily seen as bright lines. This is 

 apparently the condition which holds on the sun, as the ultra-violet hydrogen lines 

 do not show as absorption lines in the solar spectrum. 



4. Eclipse Observations of Jupiter's Satellites : a Study of the Ordinary 

 Observations in Comparison with the Photometric Observations of 

 Harvard. By Professor E,. A. Sampson. 



5. Solar Prominences and Terrestrial Magnetism} 

 By the Rev. A. L. Cortie, S.J., F.R.A.S. 



The bearing of recent researches by Father Sidgreaves, Dr. Chree, and the 

 writer upon the question of the relation that exists between sun-spots and terres- 

 trial magnetism has been to emphasise the general connection of the phenomena, but 

 to disprove any connection of efficient cause and effect. The results of the 



' Published in full in the Aitrojihysical Journal, November 1903. 



