Dr. Bowditch, President of the American Academy. xxxvii 
small or great distance from the Sun, and when approaching so near 
to a planet as to have their orbits wholly changed; and, lastly, their 
masses, which, from the circumstance of their action upon the 
planets being insensible, must be “excessively small.” In the case 
of one of them, the first comet of 1770, La Place observes, we are 
sure, that the mass is not “one five-thousandth part of that of the 
earth.” He adds, as a result of his calculations, that this comet 
passed directly through the space where Jupiter and his satellites 
were then situated ; and yet it does not appear, that the comet pro- 
duced the slightest alteration in the motions of those bodies ; and, 
he concludes, generally, that, if in the immensity of past ages some 
of the comets have encountered the planets and their satellites, 
which is very probable, it does not seem that the shock can have 
had much influence on their motions. But he remarks further, that, 
if a comet, with a mass equal to that of the Moon, should encounter 
the Moon or a satellite of Jupiter, there is not the least doubt, that 
it would render the orbit very eccentric.* 
After thus investigating the phenomena of the different bodies 
composing our system, and other parts of the universe, La Place 
proceeds to a subject intimately connected with all of them, — the 
subject of Light, and the theory of Astronomical Refractions, which 
will be further noticed hereafter. 
In a supplement to this tenth book, La Place, after referring to a 
preceding part of his work (where he had considered the phenomena 
arising from the refractive power exerted by bodies upon light) and 
stating that this force is the result of the attraction of their particles, 
remarks, that the Jaw of this attraction cannot be determined by the 
phenomena, because the only condition required, is, that it must 
become insensible at sensible distances; and this is the only case 
* Bowditch’s La Place, Vol. IV. pp. 435 — 437. 
