on the Variability and Periodic Nature of a Orionis. 315 



effect. The direct measure of the solar radiation, too, by the actino- 

 meter*, ought by no means to be neglected in this inquiry. 



" M. Poisson, in a late memoir, has considered the possible con- 

 sequences, in a geological point of view, of the sun and solar system 

 having, in long by-gone ages, passed through a region in which 

 the actual temperature of space should be much greater than in its 

 present locality f. The great authority justly attributed to every 

 idea thrown out by this philosopher, must render it a matter of dif- 

 fidence and difficulty to maintain a contrary view. Without, how- 

 ever, as a matter of abstract speculation, denying this possibility, 

 I would observe that the temperature at any given point of space 

 can arise only from two sources : 1st, That of the aether, as a fluid 

 susceptible of increase and diminution of temperature ; and, 2ndly, 

 The radiation of the stars. Of the temperature of the aether as a 

 fluid, I confess I have no conception. Of the existence of such a 

 fluid as the efficient cause of light, we have demonstrable evidence. 

 But the properties of heat are so linked and interwoven with those 

 of light, that it is asking more than can be granted to demand our 

 admission that the aether is a fluid capable of being heated and cooled, 

 while it is yet undecided (with a leaning to the affirmative side) 

 whether it be not the efficient cause of heat itself. 



" As regards the radiation of the stars. — There is a region in the 

 heavens where starlight is decidedly more dense than elsewhere — 

 the milky way. And we have, I may almost say, ocular evidence 

 that our system is excentrically situated within that zone, and 

 nearer to its southern than to its northern portion. Granting a 

 perfect transparency of the celestial spaces, the brightness of any 

 given region of the sky must be alike at all distances, whether we 

 conceive that brightness to be uniformly diffused over its surface, 

 or to emanate from a finite number of undistinguishably small 

 points. Now, although the brightness of the southern regions of 

 the milky way may, for argument's sake, be admitted to be three 

 or four times that of the northern, yet, as that light is almost com- 

 pletely obliterated by the presence of a full moon in any part of 

 the sky above the horizon, it follows that the brightness of the 

 general firmament to a spectator placed within the brightest part of 

 the milky way (supposing him not within the range of an individual 

 sun), must be less than that of {not the full moon itself, but) that 

 general illumination which the moon communicates to the whole 

 sky by atmospheric reflexion ; i. e. an almost infinitesimal quantity 

 compared to the direct light of the lunar disc, the intensity of which 

 can hardly be to that of the sun in a higher ratio than one to half 

 a million. 



" The brightest regions in the sky — i. e. the brightest spaces 



* " This instrument was devised by me for the dynamical measure of 

 the solar radiation in the spring of 1824; and I have had it in use ever 

 since, with continually increasing confidence in its indications." [See 

 p. 78 of the present volume. — Edit.] 



[f A Translation of M. Poisson's memoir will be found in the Scien- 

 tific Memoirs, vol. i. p. 122. — Edit.] 



