The effect of refraction, in respect to _terrestrial_ objects, is likewise of a beneficial nature. The quantity of this refraction is estimated by Dr. Maskelyne at one-tenth of the distance of the object observed, expressed in degrees of a great circle. Hence, if the distance be 10,000 fathoms, its tenth part 1000 fathoms, is the sixtieth part of a degree, or one minute, which is the refraction in altitude. Le Gendre estimates it at one fourteenth; De Lambre at one eleventh; and others at a twelfth of the distance; but it must be supposed to vary at different times and places according to the varying state of the atmosphere. This refraction, as it makes objects appear to be raised higher than they really are, enlarges the extent of our landscapes, and enables us to perceive distant objects which would otherwise have been invisible. It is particularly useful to the navigator at sea. It is one important object of the mariner when traversing his course, to look out for capes and headlands, rocks and islands, so as to descry them as soon as they are within the reach of his eye. Now, by means of refraction, the tops of hills and the elevated parts of coasts, are apparently raised into the air, so that they may be discovered several leagues further off on the sea than they would be, did no such refractive power exist. This circumstance is therefore a considerable benefit to the science of navigation, in enabling the mariner to steer his course aright, and to give him the most early warning of the track he ought to take, or of the dangers to which he may be exposed. In short, the effects produced by the refraction and reflection of light on the scenery connected with our globe, teach us that these principles, in the hand of the Almighty, might be so modified and directed, as to produce the most picturesque, the most glorious and wonderful phenomena, such as mortal eyes have never yet seen, and of which human imagination can form no conception; and in other worlds, more resplendent and magnificent than ours, such scenes may be fully realized, in combination with the operation of physical principles and agents, with which we are at present unacquainted. From what we already know of the effects of the reflection and the refraction of light, it is not beyond the bounds of probability to suppose, that in certain regions of the universe, light may be reflected and refracted through different mediums, in such a manner, as to present to the view of their inhabitants the prominent scenes connected with distant systems and worlds, and to an extent, as shall infinitely surpass the effects produced by our most powerful telescopes. CHAPTER III. ON THE REFRACTION OF LIGHT THROUGH SPHERICAL TRANSPARENT SUBSTANCES, OR LENSES.