THEORY OF EMISSION — DIFFICULTIES. 145 



aberration. But it might be inferred from the equality of the refraction ■whicli 

 all lig'bt, whether natural or artilicial, undergoes in passing from medium to 

 medium. Now, if light be material, it must be regarded as subject, like all other 

 projectiles, to retardation by the gravitating power of the body from which it is 

 emitted. And, moreover, it is a phenomenon inconceivable that so perpetual a 

 shower of projectiles, so infinite in number, should all be thrown with the same 

 initial velocity, and that this initial velocity should be the same for every source. 

 The only hypothesis upon which it is ])ossible to meet this last objection is to 

 assume, according to a suggestion of Jlr. Arago, that the eye is insensible to 

 luminous impressions, except for a certain definite velocity of the Inminiferous 

 particles, or for that narrow range of variation of velocity, within which arc 

 embraced the velocities which we attribute to the different colors in refracting 

 media. 



In regard to the retardation of the particles by the attracting power of the 

 luminous body itself, it may be observed that, with our ])resent means of 

 measurement, this would not be appreciable for distances so small as that which 

 separates ns from the sun, or even for distances no greater than the extreme 

 dimensions of the solar system ; at least without supposing an enormous increase 

 in ,thc mass of the luminous body beyond that of any aggregated form of matter 

 known to us. An attracting body can destroy, in a projectile thrown from it, 

 no greater an amount of velocity than it can impart to a material mass falling 

 toward it. And this limit is reached if we supjiose the falling body to commence 

 its motion at an infinite distance. Now, the velocity acquired by a body falling 

 from an infinite distance to the sun's surface, under the influence of solar at- 

 traction, would be less than four hundred miles (391 miles) per second; and of 

 tliis velocity about fourteen-fifteenlhs (365.1 miles) would be acquired after 

 passing the limit of the earth's orbit. But the body would be twenty-seven 

 and a half days in reaching the sun after passing this limit, while light is only 

 eight minutes and thirteen -seconds in traversing the same immense space. The 

 effect of an accelerating or retarding force being as its time of action, and, in 

 this case, the two times to be compared being in the ratio of about one to four 

 hundred and eighty, it may easily be shown that the retardation of light by 

 lolar attraction, during its transit from the sun to the earth, could not be so 

 much as a mile per second in its velocity. 



But the light of stars coming from distances so vast as to require years, and 

 many years, to reach us, must undergo such retardation as to render aberra- 

 tion a phenomenon exceedingly variable, unless we admit Mr. Arago's assump- 

 tion just mentioned in regard to the sensibility of the retina. Moreover, iu 

 ' cases in which the rays, in their long travel, had become reduced to velocities 

 coimaratively moderate, the gravitating power of heavy bodies near Avhich they 

 mi^t pass, ought to produce a sensible deflection of their course, and. modify, in 

 a remarkable manner, the phenomena of occultations. Nothing of this kind is 

 observed. It is here assumed that there* mny be suns much more massive tlian ours. 



Laplace has examined the question, what ought to be the mass of a luminous 

 body, in order that its gravitating power may be great enough to destroy the 

 velocity of the particles of light entirely, at some distance less than infinite — 

 the initial velocity being assumed to be that which observation has determined 

 in the sunlight as it reaches us. The expression for the velocity acquired in 

 falling i'vom an infinite distance to the sun's surface — his mass being assumed 

 to be unaltered, is — 



fimgr^ 



V Iv 



in which m is the sun's mass, that of the earth being unity; g is the measure 

 of the force of gravity at the earth's surfitce, being the velocity it is capable of 

 imparting in one second, or 321 feet; r is the earth's radius, and R the radius 

 10 s 



