October 23, 1891.] 



SCIENCE. 



23' 



ture, but also by the force of gravity in these regions: tins 

 force will be fixed by the star's mass and its stage of con- 

 densation, and will become greater as the star continues to 

 condense. 



In the ease of the sun the force of gravity has already be- 

 come so great at the surface that the decrease of the density 

 of the gases must be extremely rapid, passing in the space of 

 a few miles from atmospheric pressure to a density inSnites- 

 imally small; consequently the temperature-gradient at the 

 surface, if determined solely by expansion, must be ex- 

 tremely rapid. The gases here, however, are exposed to the 

 fierce radi ition of the sun, and unless wholly transparent 

 would take up heat, especially if any solid or liquid parti- 

 cles were present from condensation or convection currents. 



From these causes, within a very small extent of space at 

 the surface of the sun, all bodies with which we are ac- 

 quainted should fall to a condition in which the extremely 

 tenuous gas could no longer give a visible spectrum. The 

 insigniBcance of the angle subtended by this space as seen 

 from the earth should cause the boundary of the solar at- 

 mosphere to appear defined. If the boundary which we see 

 be that of the sun proper, the matter above it will have to 

 be regarded as in an essentially dynamical condition^ an 

 assemblage, so to speak, of gaseous projectiles for the most 

 part falling back upon the sun after a greater or less range 

 of flight. But in any case it is within a space of relatively 

 small extent in the sun, and probably in the other solar 

 stars, that the reversion which is manifested by dark lines is 

 to be regarded as taking place. 



Passing backward in the star's life, we should find a grad- 

 ual weakening of gravity at the surface, a reduction of the 

 temperature-gradient so far as it was determined by expan- 

 sion, and convection currents of less violence producing less 

 interference with the proportional quantities of gases due to 

 their vapor densities, while the effects of eruptions would be 

 more extensive. 



At last \ve might come to a state of things in which, if the 

 star were hot enough, only hydrogen might be sufficiently 

 cool relatively to the radiation behind to produce a strong 

 absorption. The lower vapors would be protected, and might 

 continue to be relatively too hot for their lines to appear 

 very dark upon the continuous spectrum; besides, their lines 

 might be possibly to some extent efi'aced by the coming in 

 under such conditions in the vapors themselves of a continu- 

 ous spectrum. 



In such a star the light radiated towards the upper part of 

 the atmosphere may have come from portions lower down of 

 the atmosphere itself, or at least from parts not greatly hot- 

 ter. There may be no such great difference of temperature 

 of the low and less low portions of the star's atmosphere as 

 to make tlie darkening effect of absorption of the protected 

 metallic vapors to prevail over the illuminating effect of 

 their emission. 



It is only by a vibratory motion corresponding to a very 

 high temperature that the bright lines of the first spectrum 

 of hydrogen can be brought out, and by the equivalence of 

 absorbing and emitting power that the corresponding spec- 

 trum of absorption should be produced ; yet for a strong ab- 

 sorption to show itself, the hydrogen mtist be cool relatively 

 to the source of radiation behind it, whether this be con- 

 densed particles 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 conceiving 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 condensa- 

 tion. 



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 improba- 

 bly 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 experi- 

 ment, and presumably at temperatures beyond, the maxi- 

 mum 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 ab- 

 sorption suggests that the vapors 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 particles, if we 

 could compare select parts of the continuous spectrum be- 

 tween 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 corresponding 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 

 color. 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 F to be 750 times greater than for the 

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

 suggested in 1864, and have since shown by direct observa- 

 tion, that the spectrum of the nebula in Andromeda, and 

 presumably of similar nebulae, is, in appearance, only want- 

 ing in the red. 



The stage at which the maximum radiation is in the green, 

 corresponding to the eye's greatest sensitiveness, would be 

 that in which it could be most favorably measured by eye- 

 photometry. As the maximum rose into the violet and be- 

 yond, the star would increase in visual brightness, but not 

 in proportion to the increase of energy radiated by it. 



The brightness of a star would be affected by the nature 

 of the substance by which the light was chiefly emitted. In 

 the laboratory, solid carbon exhibits the highest emissive 

 power. A stellar stage in which radiation comes, to a large 

 extent, from a photosphere of the solid particles of this sub- 



