ON PHYSICAL OPTICS." .441 



exhibit more or less of their painted sides, as they passed through their dif- 

 ferent angular positions: and the only further alteration, that could be pro- 

 duced in any of the tints, would be derived from the different degrees of light ,_^ 

 only. The same effect may also be exhibited by mixing the colours in differ- - 

 ent proportions, by means of the pencil, beginning from three equidistant 

 points as the centres of the respective colonrs, (Plate XXIX. Fig. 427.) ..j*. 



The ordinary atmospherical refraction cannot be determined in the usual 

 manner from the knowledge of its density, and of the angular direction of. the 

 incident or refracted light, since the constitution of the atmosphere is such, 

 that its density varies every where Avith its height, and the curvature of the 

 earth's surface causes the inclination of the strata through which the ray 

 passes to be perpetually changed; the difference of temperature at different 

 elevations increases also the difficulty of an exact calculation, and it is only 

 very lately that Mr. Laplace, by a comparison of astronomical with meteorologi- 

 cal observationSjhas given a satisfactory solution of the problem in all its extent. 

 But for practical uses, the refraction may be determined with sufficient 

 accuracy by an approximation which is easily remembered; the deviation 

 being at a^^l altitudes one sixth part as great as the refracted ray would 

 undergo, at the horizontal surface of a medium six times as dense as the air. 

 When a celestial object appears exactly in the horizon, it is actually more 

 than half a degree below it, since tlie refraction amounts to 33 minutes, 

 when the barometer stands at 29-^ inches, and Fahrenheit's thermometer 

 at 50'. ' * ■ 



The accidental variations of the temperature of the air, at different paits. 

 produce, however, great irregularities in its refraction, especially near thfe 

 horizon. The most remarkable of these is occasioned by the rarefac- 

 tion of the air in the neighbourhood of the surface of wrater,' of a building* 

 or of the earth itself, in consequence of which a distant object appears to be - 

 depressed instead of being elevated, and is sometimes seen at once both de- 

 pressed and elevated, so as to appear double, one of the images being gene- 

 rally in an inverted position, as if the surface possessed a reflective powei ; ~ 

 and there seems indeed to be a considerable analogy between this kind of refrac- 

 tion and the total reflection which happens within a denser medium. These 

 effects arc known by the appellations looming, mirage, and Fata Morgana: 



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