OF tHE PRODUCTION OF COLOURS. 



63i 



are smaller. It is obvious that they are the 

 immediate consequences of the coincidence 

 of a number of fringes of the same size, 

 which, as the, fibres are arranged in all ima- 

 ginable directions, must necessarily surround 

 the luminous object at equal distances on all 

 sides, and constitute circular fringes. 



There can be little doubt that the coloured 

 atmospherical halos are of the same kind ; 

 their appearance must depend on the exist- 

 ence of a number of particles of water, of 

 equal dimensions, and in a proper position, 

 with respect to the luminary and to the eye. 

 As there is no natural limit to the magnitude 

 of the spherules of water, we may expect 

 these halos to vary without limit in their dia- 

 meters"; and, accordingly, Mr. Jordan has 

 observed that their dimensions are exceed- 

 ingly various, and has remarked that they 

 frequently change during the time of obser- 

 vation. Mr. Jordan supposes that they de- 

 pend on the joint effect of two neighbouring 

 drops; but it has been shown that a unifor- 

 mity of dimensions is necessary for their pro- 

 duction, and no such uniformity can possibly 

 exist in the distances of the drops from each 

 other. 



The lines, which are seen within the shadow 

 of a hair, are produced nearly in the same 

 manner as these colours of fibres, or rather 

 they are the beginning of the series, derived 

 from two portions of light inflected into the 

 shadow, instead of one inllected and one re- 

 flected portion. 



1 first noticed the colours of mixed plates, 

 in looking at a candle through two pieces of 

 plate glass, with a httle moisture between 

 them. I observed an appearance of fringes 

 resembling the common colours of thin 

 plates; and, upon looking for the fringes 



by reflection, I found that these new fringes 

 were always in the same direction as the other 

 fringes, but many times larger. By examin- 

 ing the glasses with a magnifier, I perceived 

 th.-jt whei'ever these fringes were visible, the 

 moisture was intermixed with portions of air, 

 producing an appearance similar to dew. I 

 then supposed that the origin of the colours 

 was the same as that of the colours of halos; 

 but, on a more minute examination, I found 

 that the magnitude of the portions of air and 

 Wfiter was by no means uniform, and that 

 the explanation was therefore inadmissible. 

 It was, however, easy to find two portions of 

 light sufficient for the production of these 

 fringes; for, the light transmitted through 

 the water, moving in it with a velocity dif- 

 ferent from that of the light passing through 

 tlie interstices filled onlj' with air, the two 

 portions would interfere with each other, and 

 produce effects of colour according to the 

 general law. The ratio of the velocities, in 

 water and in air, is that of 3 to 4; the fringes 

 ought therefore to appear where the thickness 

 is 6 times as great as that which corresponds 

 to the same colour in the common case of 

 thin plates ; and, upon making the experi- 

 ment with a plain glass and a lens slightly 

 convex, I found the sixth dark circle actually 

 of the same diameter as the first in the new 

 fringes. The colours are also very easily pro- 

 duced, when butter or tallow is substituted 

 for water; and the rings then become smaller, 

 on account of the greater refractive density 

 of the oils : but, when water is added, so as to 

 fill up the interstices of the oil, the rings are 

 very mucfi enlarged ; for here the diff"erence 

 only of the velocities 'n\ water and in oil is to 

 be considered, and this is much smaller thaa 

 the diifereuce between 'an- and water. 



