THE RANGE OF VISION.

Timaru Herald, Volume XLIII, Issue 3533, 25 January 1886, Page 3

THE RANGE OF VISION.

(La JS'itturc.) In perfectly clear weather tho distance from which an object of small size (a man for example) is visible to the naked eye cannot exceed about three and a half miles, when the object i 3 seen m relief against a white background. It may bo said that an object is only visible when its movement is perceptible to tho naked eye. From Cnpo Hode, situated near Havre, it ia possible to perceive a fisherman or hunter walking upon tho sand banks of the Seine. From tho valley of tho Grindelwald, m Switzerland, it is possible to see a tourist upon the Jungfrau or Wotterhorn. This distance can bo increased by means cf a simple opera glass. From tho Havre jetty persons may thus be seen m very clear weather, upon the beach at Trouville, at a distance of eight and a half milea. I say see, and not recognise ; nnd yet certain fisherwomen of Villervillo, havo found it possible to recognise their husbands' boat starting from Havro at six miles distance. At the last shooting match at Havre, Messrs Bigot and Pelot each made a series of bull's eye 3nt nine hundred yards distance — a fact that, besides great skill, supposes a vory strong sight. In tropical seas captains agree m saying that from tho deck of their ships the distance that is visiblo around them is six leagues. In temperate zones the distance is less. In many cases the earth's curvature presents an obstacle to the rango of vision, and yet this does not always: appear to be so. Captain Duclos, of Havre, tells me that once, while becalmed off Madeira, he could not porceivo the island during tho day, but m the morning nnd evening observed it at a distance of twenty-two leagues. Professor Morel, of Paris, informs mo that while at Nice ho perceived the profile of the mountains of Corsica and even made a» sketch of it. I myself whilo near Dungeness, England, saw houses that wore situated m the environs of Boulogne-sur-Mer, Franco. In fine weather we cannot see Lion or Luc-snr-Mer from the Havre jetty, and yet thero are days when both these localities become visible. I havo found while on a steamer m ordinary weather that the said localities become visible at the middle of the route. On returning, tho electric lights of the Havre jetty become visible at the same point. There aro, then, days on which, m certain weather, and despite the curvature of the earth, the distance of visibility ia doubled or more than doubled. The phenomenon is explained to us by a well known law of physics. If wo throw a coin into an empty vessel, it will bo hidden by the edgo of tho latter, at a certain distance ; but if we fill the vessel with water, the coin will become visible. We have here a case of refraction, such as is explained m elementary works on physics. Everyone also knows the theory of tho mirage put forth by tlw celebrated Mongo, who explains the phenomenon as due to a diminution of the air's density m contact with tho superheated earth. A contrary theory will explain tho mirage at soa. Whilo the sun is making the atmosphere intensely hot, tho sea and the strata of air m contact therewith remain relatively cold, and these strata become superposed m the order of their density. A luminous ray emanating from tho sea will pass from ono stratum into another of less density, there will be a refraction and an object beneath the horizon will be seen above it. This theory supposes two conditions, to wit, a heating of the atmosphere, while the sea and earth remain relatively cold, and a calmness of it to permit its strata to become superposed m the order of their density. It is precisely when these two conditions are fulfilled that the phenomenon of the mirage occurs. On Buch days ships rise instead of descending m measure as they recede. As the horizon is perceived by direct risibility, a ship is finally seen above the horizon, as if it were suspended m the air. It is the famous "phantom ship," familiar to Bailors — a visible ship m its (natural, upright position. If atmospheric conditions were favorable, a second ship would bo aeon abovo the first, &o. Certain mariners worthy of credence have assured me that they have seen as many as eeven superposed ships,