OPTICS. 



319 



zling effect of the sun's rays, the haloes which sur- 

 round his disk may be seen to most advantage by 

 reflection in a pool of water. These phenomena are 

 attributed to the crystals of ice and snow floating in 

 the atmosphere, and, in some cases, to the action of 

 'drops of rain of different sizes. The elevation of 

 coasts, ships, and mountains above their usual level, 

 when seen in the dsitant horizon, has been long known 

 and described under the name of looming. The 

 name of mirage has been applied by the French to 

 the same class of phenomena ; and the appellation of 

 fata morgana has been bestowed by the Italians to 

 the singular appearances of the same kind, which 

 have repeatedly been seen in the straits of Messina. 

 When the rising sun throws his rays at an angle of 

 45 on the sea of Reggio, and neither wind nor rain 

 ruffle the smooth surface of the water in the bay, the 

 spectator, on an eminence in the city, who places 

 his back to the sun and his face to the sea, observes, 

 as it were upon its surface, numberless series of 

 pilasters, arches, and castles distinctly delineated ; 

 regular columns, lofty towers, superb palaces, with 

 balconies and windows ; extended valleys of trees, 

 delightful plains, with herds and flocks ; armies of 

 men on foot and horseback, and many other strange 

 figures, in their natural colours and proper actions, 

 passing one another in rapid succession. When 

 vapours and dense exhalations, rising to the height 

 of about twenty feet, appear, then the same objects 

 are seen depicted, as it were in the vapour, and 

 suspended in the air, though with less distinctness 

 than before. Captain Scoresby, when navigating 

 the Northern seas, was able to recognise his father's 

 ship when below the horizon, from the inverted 

 image of it which appeared in the air. " It was," 

 says he, " so well defined, that I could distinguish, 

 by a telescope, every sail, the general rig of the 

 ship, and its particular character, insomuch that I 

 confidently pronounced it to be my father's ship, the 

 Fame, which it afterwards proved to be ; though, in 

 comparing notes with my father, I found that our 

 relative position at the time gave our distance from 

 one another very nearly thirty miles, being about 

 seventeen miles beyond the horizon, and some 

 leagues beyond the limit of direct vision." In the 

 sandy plains of Egypt the mirage is seen to great 

 advantage. These plains are often interrupted by 

 small eminences, upon which the inhabitants have 

 built their villages, in order to escape the inunda- 

 tions of the Nile. In the morning and evening, 

 objects are seen in their natural form and position ; 

 but where the surface of the sandy ground is 

 heated by the sun, the land seems terminated, at a 

 particular distance, by a general inundation, the vil- 

 lages beyond it appearing like so many islands in a 

 great lake, and between each village an inverted 

 image of it is seen. This optical deception has been 

 noticed from the remotest times. The prophet 

 Isaiah alludes to it, when he says, "and the 

 parched ground shall become a pool." The cause 

 of these phenomena consists in variations in the 

 refractive power of the atmosphere, which may be 

 proved by actual experiment. This has been done 

 in a variety of ways ; but we shall only mention the 

 method adopted by doctor Brewster. He held a 

 heated iron above a mass of water bounded by 

 parallel plates of glass : as the heat descended 

 slowly through the fluid, a regular variation of 

 density, diminishing from the bottom to the surface, 

 took place. On withdrawing the heated iron, and 

 putting a cold body in its place, or even on allowing 

 the air to act alone, the superficial stratum of water 

 gave out its heat so as to produce a decrease of 

 density from the surface to a certain depth below it. j 

 Through the medium thus constituted, the pheno- ! 



menon of the mirage was observable in the finest 

 manner. See Fata Morgana. 



Colours of the Atmosphere. As the earth is sur- 

 rounded with an atmosphere, varying in density from 

 the surface of the globe, where it is the densest, to 

 the height of about forty-five miles, where it is 

 extremely rare, and just able to reflect the rays of 

 the setting sun, the rays of the sun, moon, and stars 

 are refracted into curve lines, unless when they aro 

 incident upon it perpendicularly. Hence the appa- 

 rent altitude of the celestial bodies is always greater 

 than their real altitude, and they appear above the 

 horizon when they are actually below it. But while 

 the solar rays traverse the earth's atmosphere, they 

 suffer another change from the resisting medium 

 which they encounter. When the sun, or any of the 

 heavenly bodies, is considerably elevated above the 

 horizon, its light is transmitted to the earth without 

 any perceptible change ; but when these bodies are 

 near the horizon, their light must pass through a long 

 tract of air, and is considerably modified before it 

 reaches the eye of the observer. The momentum of 

 the red, or greatest refrangible rays, being greater 

 than the momentum of the violet, or least refrangible 

 rays, the former will force their way through the 

 resisting medium, while the latter will be either 

 reflected or absorbed. A white beam of light will 

 therefore be deprived of a portion of its blue 

 rays by its horizontal passage through the atmo- 

 sphere, and the resulting colour will be either 

 orange or red, according to the quantity of the least 

 refrangible rays that have been stopt in their course; 

 hence the rich and brilliant hue with which nature 

 is gilded by the setting sun, and hence the 

 glowing red which tinges the morning and evening 

 cloud. We have already seen that the red rays 

 penetrate through the atmosphere, while the blue 

 rays, less able to surmount the resistance which they 

 meet, are reflected or absorbed in their passage. It 

 is to this cause that we must ascribe the blue colour 

 of the sky, and the bright azure which tinges the 

 mountains of the distant landscape. As we ascend 

 in the atmosphere, the deepness of the blue tinge 

 dies away; and to the aeronaut who has soared 

 above the denser strata, or to the traveller who has 

 ascended the Alps or the Andes, the sky appears of 

 a deep black, while the blue rays find a ready pas- 

 sage through the attenuated strata of the atmosphere 

 It is owing to the same cause, that the diver at the 

 bottom of the sea is surrounded with the red light 

 which has pierced through the superincumbent fluid, 

 and that the blue rays are reflected from the surface 

 of the ocean. Were it not for the reflecting power 

 of the air, and of the clouds which float in the lower 

 regions of the atmosphere, we should be involved in 

 total darkness by the setting of the sun, and all the 

 objects around us would suffer a total eclipse by 

 every cloud that passed over his disk. It is to the 

 multiplied reflections which the light of the sun suf- 

 fers in the atmosphere that we are indebted for the 

 light of day, when the earth is enveloped with im- 

 penetrable clouds. From the same cause arises the 

 sober hue of the morning and evening twilight, which 

 increases as we recede from the equator, till it 

 blesses with perpetual day the inhabitants of the, 

 polar regions. 



Coloured Shadows. The shadows of bodies placed 

 only in one light, and at a distance from all other 

 bodies capable of reflecting light, must necessarily be 

 black. In a summer morning, or evening, however, 

 the shadows of bodies formed either by the light of 

 the sun, or by that of a candle, have been observed 

 to be blue : this obviously arises from the shadows 

 being illuminated with the light of the blue sky. 

 The colours thus produced vary in different coun- 



