In conformity with this law, the relative quantities of light on the surfaces of the planets may be easily determined, when their distances from the sun are known. Thus, the distance of Uranus from the sun is 1,800,000,000 miles, which is about nineteen times greater than the distance of the earth from the same luminary. The square of 19 is 361; consequently the earth enjoys 361 times the intensity of light when compared with that of Uranus; in other words, this distant planet enjoys only the 1/361 part of the quantity of light which falls upon the earth. This quantity, however, is equivalent to the light we should enjoy from the combined effulgence of 348 full moons; and if the pupils of the eyes of the inhabitants of this planet be much larger than ours, and the _retina_ of the eye be endued with a much greater degree of nervous sensibility, they may perceive objects with as great a degree of splendour as we perceive on the objects which surround us in this world. Following out the same principle, we find that the quantity of light enjoyed by the planet Mercury is nearly _seven_ times greater than that of the Earth, and that of Venus nearly _double_ of what we enjoy--that Mars has less than the one half--Jupiter the _one twenty-seventh_ part--and Saturn only the _one ninetieth_ part of the light which falls upon the Earth. That the light of these distant planets, however, is not so weak as we might at first imagine appears from the brilliancy they exhibit, when viewed in our nocturnal sky, either with the telescope or with the unassisted eye--and likewise from the circumstance that a very small portion of the Sun--such as the one fortieth or one fiftieth part diffuses a quantity of light sufficient for most of the purposes of life, as is found in the case of total eclipses of the Sun, when his western limb begins to be visible, only like a fine luminous thread, for his light is then sufficient to render distinctly visible all the parts of the surrounding landscape.