The reflection and refraction of the rays of light frequently produce phenomena which astonish the beholders, and which have been regarded by the ignorant and the superstitious, as the effects of supernatural agency. Of these phenomena I shall state a few examples.
One of the most striking appearances of this kind is what has been termed the Fata Morgana, or optical appearances of figures in the sea and the air, as seen in the Faro of Messina. The following account is translated from a work of Minasi, who witnessed the phenomenon, and wrote a dissertation on the subject. “When the rising sun shines from that point whence its incident ray forms an angle of about forty-five degrees to the sea of Riggio, and the bright surface of the water in the bay is not disturbed either by the wind or the current, the spectator being placed on an eminence of the city, with his back to the sun and his face to the sea;—on a sudden there appear on the water, as in a catoptric theatre, various multiplied objects, that is to say, numberless series of pilasters, arches, castles well delineated, regular columns, lofty towers, superb palaces, with balconies and windows, extended alleys of trees, delightful plains with herds and flocks, armies of men on foot and horseback, and many other strange images, in their natural colours and proper actions, passing rapidly in succession along the surface of the sea, during the whole of the short period of time, while the above mentioned causes remain.—But, if in addition to the circumstances now described, the atmosphere be highly impregnated with vapour and dense exhalations, not previously dispersed by the winds or the sun, it then happens that, in this vapour, as in a curtain extended along the channel, at the height of about thirty palms, and nearly down to the sea, the observer will behold the scene of the same objects, not only reflected from the surface of the sea, but likewise in the air, though not so distant or well defined, as the former objects from the sea.—Lastly, if the air be slightly hazy or opake, and at the same time dewy and adapted to form the iris, the then above-mentioned objects will appear only at the surface of the sea, as in the first case, but all vividly coloured or fringed with red, green, blue and other prismatic colours.”[11]
It is somewhat difficult to account for all the appearances here described; but, in all probability, they are produced by a calm sea, and one or more strata of superincumbent air differing in refractive and consequently in reflective power. At any rate reflection and refraction are some of the essential causes which operate in the production of the phenomena.
The Mirage, seen in the deserts of Africa, is a phenomenon, in all probability produced by a similar cause. M. Monge, who accompanied the French army to Egypt, relates that, when in the desert between Alexandria and Cairo, the mirage of the blue sky was inverted, and so mingled with the sand below, as to give to the desolate and arid wilderness an appearance of the most rich and beautiful country. They saw, in all directions, green islands, surrounded with extensive lakes of pure, transparent water. Nothing could be conceived more lovely and picturesque than the landscape. In the tranquil surface of the lakes, the trees and houses with which the islands were covered, were strongly reflected with vivid and varied hues, and the party hastened forward to enjoy the cool refreshments of shade and stream which these populous villages proffered to them. When they arrived, the lake on whose bosom they floated, the trees among whose foliage they were embowered, and the people who stood on the shore inviting their approach, had all vanished, and nothing remained but an uniform and irksome desert of sand and sky, with a few naked huts and ragged Arabs. Had they not been undeceived by their nearer approach, there was not a man in the French army who would not have sworn that the visionary trees and lakes had a real existence in the midst of the desert.
Dr. Clark observed precisely the same appearances at Rosetta. The city seemed surrounded with a beautiful sheet of water; and so certain was his Greek interpreter—who was unacquainted with the country—of this fact, that he was quite indignant at an Arab who attempted to explain to him that it was a mere optical delusion. At length they reached Rosetta in about two hours, without meeting with any water; and on looking back on the sand they had just crossed, it seemed to them as if they had waded through a vast blue lake.
figure 29.
On the 1st of August, 1798, Dr. Vince observed at Ramsgate a ship which appeared as at A, (fig. 29.) the topmast being the only part of it that was seen above the horizon. An inverted image of it was seen at B, immediately above the real ship A, and an erect image at C, both of them being complete and well defined. The sea was distinctly seen between them, as at V W. As the ship rose to the horizon the image C gradually disappeared, and while this was going on, the image B descended, but the mainmast of B did not meet the mainmast of A. The two images BC were perfectly visible when the whole ship was actually below the horizon. Dr. Vince then directed his telescope to another ship whose hull was just in the horizon, and he observed a complete inverted image of it, the mainmast of which just touched the mainmast of the ship itself. He saw at the same time several other ships whose images appeared in nearly a similar manner, in one of which the two images were visible when the whole ship was beneath the horizon. These phenomena must have been produced by the same causes which operated in the case formerly mentioned, in relation to Captain Scoresby, when he saw the figure of his father’s ship inverted in the distant horizon. Such cases are, perhaps not uncommon, especially in calm and sultry weather, but they are seldom observed, except when a person’s attention is accidentally directed to the phenomenon, and, unless he use a telescope, it will not be so distinctly perceived.
The following phenomenon, of a description nearly related to the above, has been supposed to be chiefly owing to reflection. On the 18th of November, 1804, Dr. Buchan, when watching the rising sun, about a mile to the east of Brighton, just as the solar disk emerged from the surface of the water, saw the face of the cliff on which he was standing, a windmill, his own figure and the figure of his friend, distinctly represented, precisely opposite, at some distance from the ocean. This appearance lasted about ten minutes, till the sun had risen nearly his own diameter above the sea. The whole then seemed to be elevated into the air and successively disappeared. The surface of the sun was covered with a dense fog of many yards in height, which gradually receded from the rays of the sun as he ascended from the horizon.
The following appearance most probably arose chiefly from the refraction of the atmosphere. It was beheld at Ramsgate, by Dr. Vince of Cambridge and another gentleman. It is well known that the four turrets of Dover castle are seen at Ramsgate, over a hill which intervenes between a full prospect of the whole. On the 2nd of August, 1806, not only were the four turrets visible, but the castle itself appeared as though situated on that side of the hill nearest Ramsgate, and so striking was the appearance, that for a long time the Doctor thought it an illusion; but at last, by accurate observation, was convinced that it was an actual image of the castle. He, with another individual, observed it attentively for twenty minutes, but were prevented by rain from making further observations. Between the observers and the land from which the hill rises, there were about six miles of sea, and from thence to the top of the hill there was about the same distance, their own height above the surface of the water was about seventy feet.—The cause of this phenomenon was, undoubtedly, unequal refraction. The air being more dense near the ground and above the sea than at greater heights, reached the eye of the observer, not in straight but in curvilinear lines. If the rays from the castle had in their path struck an eye at a much greater distance than Ramsgate, the probability is, that the image of the castle would have been inverted in the air; but in the present case,the rays from the turret and the base of the castle had not crossed each other.