OPTICS. 



59 



between the rays of the top and bottom 

 of the castle. 



That the phenomena of the Mirage 

 are produced by such variations in the 

 refractive power of the atmosphere as 

 we have mentioned, may be proved 

 by actual experiment : All the pheno- 

 mena may be represented artificially to 

 the eye, and we may even venture to 

 predict new phenomena which have not 

 yet been witnessed. If the variation of 

 the refractive power of the air takes 

 place in a horizontal line perpendicular 

 to the line of vision, that is, from right 

 to left, then we may have a lateral Mi- 

 rage, that is, an image of a ship may 

 be seen on the right or left hand of the 

 real ship, or on both, if the variation of 

 refractive power is the same on each 

 side of the line of vision.* If there 

 should happen at the same time both 

 a vertical and a lateral variation of re- 

 fractive power in the air, and if the 

 variation should be such as to expand 

 or elongate the object in both direc- 

 tions, then the object would be magni- 

 fied, as if seen through a telescope, 

 and might be seen and recognised at a 

 distance at which it would not other- 

 wise have been visible. If the refract- 

 ive power, on the contrary, varied, so as 

 to contract the object in both directions, 

 the image of it would be diminished as 

 if seen through a concave lens. 



In order to represent artificially the 

 effects of the Mirage, Dr. Wollaston 

 views an object through a stratum of spirit 

 of wine lying above water, or a stratum 

 of water laid above one of syrup. These 

 substances, by their gradual incorpora- 

 tion, produce a refractive power dimi- 

 nishing from the spirit of wine to the 

 water, or from the syrup to the water ; 

 so that, by looking through the mixed, 

 or the intermediate stratum at a word 

 or object held behind the bottle which 

 contains the fluids, an inverted image 

 will be seen. The same effect Dr. Wol- 

 laston has shown may be produced by 

 looking along the side of a red-hot 

 poker at a word or object ten or twelve 

 feet distant. At a distance less than 

 three-eighths of an inch from the line 

 of the poker, an inverted image was 

 seen, and within and without that an 

 erect image. 



The method employed by Dr. Brew- 

 stvrt to illustrate these phenomena con- 



* M.M. Jurine ami Soret observed a fact of this 

 V.ia.1 in the lake of Geneva. Ency- 



dopedia, Art. OPTICS, vol. xv. p 



: Edinburgh Encyclopedia, Art. HEAT, vol. x. 



sists in holding a heated iron above a 

 mass of water bounded by parallel 

 plates of glass : as the heat descends 

 slowly through the fluid, we have a 

 regular variation of density which gra- 

 dually diminishes from the bottom to 

 the surface. If we now withdraw the 

 heated iron, and put a cold body in its 

 place, or even allow the air to act alone, 

 the superficial stratum of water will 

 give out its heat, so as to produce a 

 decrease of density from the surface to 

 a certain depth below it. Through the 

 medium thus constituted, the pheno- 

 mena of the Mirage may be seen in the 

 finest manner. 



We have no doubt that some of the 

 facts ascribed in the Western High- 

 lands of Scotland to second sight, 

 have been owing to the unusual refrac- 

 tion of the atmosphere, and that the 

 same cause will explain some of those 

 wonders which sceptics discredit, and 

 which superstitious minds attribute 

 to supernatural causes. The beacon 

 keeper of the Isle of France, who saw 

 ships in the air before they rose above 

 the visible horizon, may now recover his 

 good character in the eyes of the former, 

 while the latter may cease to regard him 

 as a magician. 



4. On the Colours of Natural Bodies. 

 There are few of the applications of 

 science to explain natural phenomena 

 so extremely simple, and at the same 

 time so beautiful, as that of the co- 

 lours of thin plates, to account for all 

 that variety of splendid tints, which 

 colour the animal, the mineral, and the 

 vegetable kingdom. To Sir Isaac New- 

 ton we owe this explanation ; and we 

 have no hesitation in saying, that none 

 of his discoveries exhibit more penetra- 

 tion and sagacity. 



The colours of bodies maybe deduced 

 from those of thin plates, as explained 

 in Chap, xiii., in the following manner, 

 and without ascribing any new property 

 to the particles of matter. 



1 . Those surfaces of transparent bo- 

 dies reflect the greatest quantity of light 

 that have the greatest refractive pow ,; 

 or that separate two media which differ 

 most in their refractive power. When 

 two media have the same rcfractire 

 power, no light is reflected at their sepa- 

 rating surfaces. 



This proposition may be proved by 

 many facts : chromate of lead and dia- 

 mond, and the other bodies which are 

 placed at the head of our table of re- 

 fractive powers in Chap, ii., reflect 



