ADDRESS. 1 7 



or gas. Such conditions, it seems to me, should occur in the earlier rather 

 than in the more advanced stages of condensation. 



The subject is obscure, and we may go wrong in our mode of conceiv- 

 ing of the probable progress of events, but there can be no doubt that in 

 one remarkable instance the white-star spectrum is associated with an 

 early stage of condensation. 



Sirius is one of the most conspicuous examples of one type of this 

 class of stars. Photometric observations combined with its ascertained 

 parallax show that this star emits from forty to sixty times the light of 

 our sun, even to the eye, which is insensible to ultra-violet light, in which 

 Sirius is very rich, while we learn from the motion of its companion 

 that its mass is not much more than double that of our sun. It follows 

 that unless we attribute to this star an impi-obably great emissive power, 

 it must be of immense size, and in a much more diffuse and therefore 

 an earlier condition than our sun ; though probably at a later stage 

 than those white stars in which the hydrogen lines are bright. 



A direct determination of the relative temperature of the photospheres 

 of the stars might possibly be obtained in some cases from the relative 

 position of maximum radiation of their continuous spectra. Langley 

 has shown that through the whole range of temperature on which we can 

 experiment, and presumably at temperatures beyond, the maximum of 

 radiation-power in solid bodies gradually shifts upwards in the spectrum 

 from the infra-red through the red and orange, and that in the sun it has 

 reached the blue. 



The defined character as a rule of the stellar lines of absorption sug- 

 gests that the vapours producing them do not at the same time exert any 

 strong power of general absorption. Consequently we should probably 

 not go far wrong, when the photosphere consists of liquid or solid parti- 

 cles, if we could compare select parts of the continuous spectrum between 

 the stronger lines or where they are fewest. It is obvious that if extended 

 portions of different stellar spectra were compared, their true relation 

 would be obscured by the line-absorption. 



The increase of temperature, as shown by the rise in the spectrum of 

 the maximum of radiation, may not always be accompanied by a corre- 

 sponding greater brightness of a star as estimated by the eye, which is an 

 extremely imperfect photometric instrument. Not only is the eye blind 

 to large regions of radiation, but even for the small range of light that 

 we can see the visual effect varies enormously with its colour. According 

 to Professor Langley, the same amount of energy which just enables us to 

 perceive light in the crimson at A would in the green produce a visual 

 effect 100,000 times greater. In the violet the proportional effect would 

 be 1,600, in the blue 62,000, in the yellow,28,000, in the orange 14,000, 

 and in the red 1,200. Captain Abney's recent experiments make the 

 •sensitiveness of the eye for the green near P to be 750 times greater than 

 for red about C. It is for this reason, at least in part, that I suggested 



1891. ' C 



