OPTICS. 



heat of the sun. He then shows, by 

 calculation, that when the shadows of 

 the globule is to the radius of the 

 opaque kernel or nucleus, as 1000 to 

 480, a halo 45 in diameter will be pro- 

 duced ; and that when the proportion of 

 the same radii is as 1000 to 680, a halo 

 of 9J in diameter will be produced. 



In order to explain the more complex 

 phenomena, similar to that shown in 

 jig. 51, Huygens resorts to half thawed 

 cylinders or snow, differing; only in 

 form from the globules above described. 

 He considers the large white circle, 

 B E F D C, (fig. 51.) as formed by the 

 reflection of the sun's rays from the 

 outer surface of these cylinders, which 

 he supposes to have an upright posi- 

 tion. The lateral parhelia, B, C, he 

 ascribes to two refractions of the sun's 

 light through the watery cylinder, 

 and he regards the halos which pass 

 through the parhelia as produced by 

 the round ends of the upright cylinders. 

 He considers the inverted arches T H S, 

 Q G R, as produced by two refractions 

 in those cylinders whose axes are 

 parallel to the plane of the horizon 

 though not to one another ; the parhe- 

 lia which appear in the middle of these 

 arches being nothing else than the 

 brightest parts of them 



The subject of halos occupied like- 

 wise the attention of Sir Isaac Newton. 

 He considers the halo of 45 in diame- 

 ter as different trom the smaller pris- 

 matic ones, and as " made by refraction 

 in some sort of hail or snow" floating in 

 the air in an horizontal posture, the re- 

 fracting angle being about 58 or 60." 

 Sir Isaac explains the small prismatic 

 halos by the fits of easy reflexion and 

 transmission in small drops of water, 

 and he concludes that the rings will be 

 greater or less according as the globules 

 are greater or smaller. 



M. Mariotte and Dr. Young ascribe 

 halos to two refractions of equilateral 

 prisms of snow having angles of 60, a 

 supposition which is the more probable, 

 as ice actually crystallizes in six-sided 

 prisms. Mariotte, indeed, observed, that 

 the filaments of hoar frost had three 

 equal faces, and exhibited rainbows 

 when placed in the sun ; and he calcu- 

 lated that they would produce a halo 

 whose diameter was 4 5 40', which is 

 very near 45 50', the mean of five ac- 

 curate observations. Dr. Young ac- 

 counts for the halo of 90, by supposing 

 that a considerable portion of the light 

 may fail, after passing through one 



prism, upon a second prism, so that 

 the effect will be doubled, and a halo of 

 90 produced. Mr. Cavendish has sug- 

 gested, that this large halo may be 

 produced by the refraction of the rect- 

 angular termination of the crystals, 

 which would give a halo of 90 28', if 

 the index of refraction for ice be 1 .3 1 .* 

 3. Phenomena of the Mirage, or 

 unusual refraction. The elevation of 

 coasts, ships, and mountains above 

 their usual level, when seen in the dis- 

 tant 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 given by the 

 Italians to the singular appearances 

 of the same kind which have been re- 

 peatedly 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, dis- 

 tinctly delineated ; regular columns, 

 lofty towers, superb palaces with bal- 

 conies 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, accompany the 

 state of the atmosphere above described, 

 then the same objects are seen depicted 

 as it were in the vapour and suspended 

 in the air, though with less distinctness 

 than before. If the air be slightly hazy, 

 and at the same time dewy and fitted 

 to form the rainbow, the above-men- 

 tioned objects appear only at the sur- 

 face of the sea, but they are all bril- 

 liantly fringed with the prismatic co- 

 lours. This description of the Fata 

 Morgana, given by Antonio Minasi so 

 recently as 1793, is no doubt a little 

 overcharged, but there can be no hesi- 

 tation in believing that the objects and 

 movements which existed on the oppo- 

 site coast, were occasionally displayed in 

 all the grandeur of aerial representation. 



* Since writing the above, we have found quaclri- 

 literal prisms of ice with angles of 9U 1 ', so th:it it is 

 through thce faces that the halo of fO" is produce^. 



