182 DR. YOUNG'S ACCOUNT OF SOME CASES 



the deviation of the firft red fringe at the diftance -^ ; and, as 

 re : rih> »^: Tsi-iss* or ttsts *° r tne difference of the 

 routes of the red light where it was mod intenfe. The meafure 

 deduced from Newton's experiments is -j^o^* I thought 

 this coincidence, with only an error of one-ninth of fo minute 

 a quantity, fufliciently perfect to warrant completely the ex- 

 planation of the phenomenon, and even to render a repetition 

 of the experiment unneceflary ; for there are feveral circum- 

 ftances which make it difficult to calculate much more precifely 

 what ought to be the refult of the meafurement. 

 The halos When a number of fibres of the fame kind, for inftance, a 



candt fee)f ant uniform lock of w00, > are held near to the e )' e > we fee an 



through wool, appearance of halos furrounding a diftant candle : but their 

 brilliancy, and even their exiftence, depends on the uniformity 

 of the dimenfions of the fibres ; and they are larger as the 

 fibres are fmaller. It is obvious that they are the immediate 

 confequences of the coincidence of a number of fringes of 

 the fame fize, which, as the fibres are arranged in all imagina- 

 ble directions muft neceffarily furround the luminous object at 

 equal diftances on all fides, and conftitute circular fringes, 

 poloured atmof- There can be little doubt that the coloured atmofpherical 

 phcnc halos. na ] os are f the fame kind : their appearance muft depend on 

 the exiftence of a number of particles of water, of equal 

 dimenfions, and in a proper pofition, with refpect to the 

 luminary and to the eye. As there is no natural limit to the 

 magnitude of the fpherules of water, we may expect thefe 

 halos to vary without limit in their diameters; and, accordingly, 

 Mr. Jordan has obferved that their dimenfions are exceedingly 

 various, and has remarked that they frequently change during 

 the time of obfervation. 

 New colours I fi r ft noticed the colours of mixed plates, in looking at a 



plates of U gla(s W ° can ^ e through two pieces of plate-glafs, with a little moifture 

 with a little between them. I obferved an appearance of fringes refembling 

 ^em^nu'xed^ 6611 ^ 6 common colours of thin plates ; and, upon looking for the 

 y/itb air. fringes by reflection, I found that thefe new fringes were 



always in the fame direction as the other fringes, but many 

 times larger. By examining the glaffes with a magnifier, I 

 perceived that wherever thefe fringes were vifible, the moifture 

 was intermixed with portions of air, producing an appear- 

 ance fimilar to dew. I then fuppofed that the origin of the 

 colours was the fame as that of the colours of halos 5 but, on 



