Sentence:  Thus she [one of the moons of Mars] has a disc, always on this assumption be it remembered, equal to about a quarter of our moon's; and being illuminated by the sun, like the other moon, with a light varying from one-half to one-third that which he pours on the earth, it follows that the light she reflects to <WSD>Martians</WSD>, or would reflect to them if there were any such beings, varies from one-eighth to one-twelfth of that which we receive from the full moon. 
Meaning of the word marked with <WSD> tags:  An inhabitant of the planet Mars.