The fact that the Earth is surrounded by a considerable atmosphere largely composed of aqueous vapour has a material bearing on the success or failure of observations made on the Earth of bodies situated at a distance. It may be taken as a general rule that the nearer an observer is to the surface of the sea, or otherwise to the surface of the land at the sea-level, the greater will be the difficulty which will confront him in carrying on astronomical observations. Hence such observations are generally made with unsatisfactory results on the sea coast or on the banks of rivers. An interesting but rather ancient illustration of this last-named fact is to be found in the circumstance that Copernicus, who died at the age of 70, complained in his last moments that much as he had tried he had never succeeded in detecting the planet Mercury, a failure due, as Gassendi supposed, to the vapours prevailing near the horizon at the town of Thorn on the banks of the Vistula where the illustrious philosopher lived. The phenomena depending on the presence of aqueous vapour in the atmosphere which especially under the notice of the astronomer are Refraction, Twilight, and the Twinkling of the Stars. Refraction is what it professes to be, a bending, and what is bent is the ray of light coming from a celestial object to a terrestrial station. Olmsted has put the matter in this way:—“We must consider that any such object always appears in the direction in which the _last_ ray of light comes to the eye. If the light which comes from a star were bent into fifty directions before it reached the eye, the star would nevertheless appear in a line described by the ray nearest the eye. The operation of this principle is seen when an oar, or any stick, is thrust into the water. As the rays of light by which the oar is seen have their direction changed as they pass out of water into air, the apparent direction in which the body is seen is changed in the same degree, giving it a bent appearance—the part below the water having apparently a different direction from the part above.” The direction of this refraction is determined by the general law of optics that when a ray of light passes out of a rarer into a denser medium (for instance out of air into water, or out of space into the Earth’s atmosphere) it is bent _towards_ a perpendicular to the surface of the medium; but when it passes out of a denser into a rarer medium it is bent _from_ the perpendicular. The effect of refraction is to make a heavenly body appear to have an apparent altitude greater than its true altitude, so that, for example, an object situated actually _in_ the horizon will appear _above_ it. Indeed it sometimes happens that objects which are actually below the horizon and which otherwise would be invisible were it not for refraction are thus brought into sight. It was in consequence of this that on April 20, 1837, the Moon rose eclipsed before the Sun had set.