*590 



SCIENCE. 



On a curi- 

 ous case of 

 mirage. 



Science, ed back to the retina by the choroid coat will not con- 

 Curiosities ta j n tnat O f re( j light.* See Dr. Brewster's Journal of 

 _^^j Science, No. VII. 



5. On a Curious Case of Mirage. 

 In our article OPTICS, Vol. XV. p. 617, we have 

 discussed the subject of mirage at great length, since 

 that article was printed the following curious pheno- 

 menon has been described by Mr. H. H. Blackadder. 

 The bulwark of St. George's bastion, to the north-east 

 of the new docks of Leith, is formed of huge blocks 

 of cut sandstone. From the solid stone tower east- 

 ward, the bulwark forms a straight line for the length 

 of about 498 feet. It is eight feet high on the face 

 next the land> and it has a foot way three feet above 

 the ground, and upwards of two feet broad. The 

 parapet at the top is three feet wide, and inclines great- 

 ly to the sea. 



In weather favourable to the production of the 

 mirage, which is by no means of rare occurrence, the 

 top of the parapet resembles a mirror or a sheet of ice : 

 and when this happens, any person standing or walk- 

 ing upon it, will appear to an observer at a little dis- 

 tance to be accompanied with an inverted image seen 

 under him. If, when the observer stands on the 

 footway, another person stands on it also, with his 

 face turned towards the sea, his image will appear op. 

 posite to him, exhibiting the appearance of two per- 

 sons talking together or saluting each other. If the 

 observer, when standing on the footway, looks along 

 the parapet to the east, another person crosses the 

 eastern extremity of the bulwark, passing through 

 the Watergate, either to or from the sea, there will 

 be produced the appearance of two persons moving 

 in opposite directions, constituting what has been call- 

 ed lateral mirage.t The first figure is seen moving 

 past, and then the other in an opposite direction, with 

 some interval between them. In looking over the pa- 

 rapet, the distant objects are seen variously modified, 

 the hills in the county of Fife being converted into 

 immense bridges. 



If the observer now stations himself at the east end 

 of the bulwark, and directs his eye to the tower, the 

 latter will appear to be curiously modified, part of it 

 being as it were cut off, and brought down so as to re- 

 PLATE semble another small and elegant tower, as shown in 



CCCCLXXXVI. Plate CCCCLXXXVI. Fig. 16. At other times the 

 Fig. 16. summit of the tower resembles an ancient altar, the 

 spire of which seems to burn with great intensity. 

 At some distance beyond the tower, there is seen the 

 chimney-top of a house for boiling pitch, or for other 

 purposes connected with the docks. When the smoke 

 'issues from the chimney, the appearance shown in 

 Fig. 17. Fig. 17- is produced. The black waved lines beneath 

 the smoke had a rapid vibrating motion, while the mo- 

 tion of that which represents the fire of the altar resem- 

 bled exactly, with the exception of colour, the flame 

 of a strong fire. 



Although the phenomenon now described is local, 

 yet the reader can have no difficulty in discovering si- 

 milar places where similar phenomena may be observ- 

 ed, when the state of the atmosphere is favourable to 

 the production of such appearances. See the Edin- 

 burgh Journal of Science, vol. iii. p. 13. 



6. Method of forming three Haloes artificially round 



the Sun, or any luminous object. 

 If we spread a few drops of a saturated solution of 



alum over a plate of glass, it will speedily crystallize, 

 covering the glass with an imperfect crust, consisting 

 of flat octahedral crystals, scarcely visible to the eye. 

 When this plate is held between the sun, or any other 

 luminous body, and the observer, whose eye must be 

 placed very close to the smooth side of the glass plate, 

 he will see three fine haloes surrounding the luminous 

 body at different distances. The innermost halo, which 

 is the whitest, is formed by the refraction of the rays 

 of the sun, through the pair of faces of the octo- 

 hedral crystals, not much inclined to each other. The 

 second halo, which is more coloured, with the blue rays 

 outwards, is formed by refraction through a pair of 

 faces more inclined to each other ; and the third halo, 

 which is very large and highly coloured, is formed by 

 a still more inclined pair of faces. 



Each separate crystal of the alum forms three images 

 of the sun, placed at points 120 degrees distant from 

 one another, in a circle of which the sun is the centre ', 

 and as the numerous crystals have their refracting faces 

 turned in every possible direction, as they lie on the 

 glass plate, the whole circumference of each halo 

 will be completely filled up. IC The same effects," 

 says Dr. Brewster, who first made this experiment, 

 " may be obtained with other crystals ; and when they 

 have the property of double refraction, (which alum 

 has not,) each halo will be either doubled when 

 the double refraction is considerable, or rendered 

 broader, or otherwise modified in point of colour when 

 the double refraction is small. The effects may be cu- 

 riously varied by crystallizing on the same 'plate of 

 glass crystals of a decided colour, by which means we 

 should have white and coloured haloes succeeding each 

 other. 



7. On Mr. Barton's Iris Ornaments. 



Mr. John Barton of the Mint, whose mechanical ta- On Mr. 

 lents are well known, has recently taken out a patent Bart on's 

 for a method of ornamenting steel and other sub- !s Orna- 

 stances, by covering their surface with a great number ments - 

 of minute lines or grooves, which, though invisible to 

 the eye individually, produce over the whole surface of 

 the body the most brilliant prismatic colours. These 

 minute lines or grooves are drawn in a number 

 of directions, so as to form a pattern of great beauty. 

 They are cut with the point of a diamond exactly 

 parallel to each other, by means of a small engine given 

 to him by his father-in-law, the late celebrated Mr. 

 Harrison ; and so sure is its operation, that by means 

 of it he can draw 10,000 in an inch ; and if, in draw- 

 ing 2000 in an inch he omits one line intentionally, he 

 can, after taking off the plate, restore it to its place, 

 and introduce the line without its being distinguish- 

 able from the rest. 



When the light of the sun or of a candle is reflected 

 to the eye from a plate of steel covered with these 

 grooves, the image of the sun or candle seen by ordi- 

 nary reflexion, has on each side of it a series of pris- 

 matic images of the sun or candle, those nearest the 

 common image resembling those produced by a prism 

 of a less refracting angle than those which are more 

 remote. The pair of images nearest the common 

 image are more distant from each other, and the re- 

 fraction of the colours more complete the closer that 

 the grooves are to each other. When, for example, 

 there are 4000 grooves in an inch, the refraction of 



* This opinion is founded on a theory .of vision which has not yet been published, 

 f See OPTICS, Vol. XV. p. 620, col. 1. 



