To understand clearly the variable duration of the day at all points of the terrestrial surface, let us so place ourselves as to see our planet distinctly in front. I need hardly say that this invitation is not addressed to the visual organs of the body, but to the "mind's eye, Horatio." With the former, fixed as we are on the surface of the globe, we can only see the shadows of objects standing out in relief upon it, and still, with the sun at our back, can perceive but a comparatively insignificant portion. With the second, on the contrary, we may detach ourselves from the earth, may mount afar into space, may plant ourselves on some aërial height, whence we may embrace, at a single glance, all the illuminated hemisphere of our globe: we shall have but a single shadow on the prospect,--that which our brightened planet casts behind it, and which, in four-and-twenty hours, traverses the entire surface of the globe, producing light wherever it passes. * * * * * Having completed our preparations, let us note what we perceive with our eye thus disjoined from our body,--an eye equally well adapted to fix every moving object, and to see at any distance bodies extremely small, or bodies extremely large. What a marvellous eye! And it is for us to develop its power, and freely to increase its range. What an arduous task! The luminous envelope, the photosphere of the sun, simultaneously darts its rays in all directions, with an intensity which diminishes with the square of the distance. Let us follow those which direct their course towards our planet. So long as the rays do not come into collision, or strike against resistant matter, they neither warm nor enlighten. Confirming the Newtonian hypothesis of emission, they travel straight as arrows through the icy space, the shadowy ocean in which the waves of the terrestrial atmosphere are lost. But the moment they encounter a material obstacle, the rays partly penetrate it, and are partly thrown back, in such a manner as to form a series of undulations like those which the falling of a stone into a pond produces. As they recede from these repeated shocks, the rays come under that theory of undulation which Huygens promoted.[57]