Phenomena of the Rainbow. 287 



li^lit twice refracted and once or twice reflected. The drops 

 which are not thus made sensible, are those contained in the 

 space subtending the angle between the red edges of the pri- 

 mary and secondary bows, which space is therefore compara- 

 tively dark, and constitutes the hitherto unexplained part of the 

 phenomena. The rainbow, it will have been observed in the 

 account of M. Biot, and I believe in all others, is described 

 as an insulated coloured band, rendered visible by the paral- 

 lelism of the rays, which emerge under a particular angle of 

 incidence, without any allusion to the illuminated space, of 

 which it is the coloured edge ; the colour being nearly analo- 

 gous to the coloured edges given to luminous objects when 

 viewed through lenses or prisms. The parallelism of the rays 

 at the maximum angle of deviation adds greatly to the bril- 

 liancy of the bow, as compared with the other parts of the 

 illuminated space, and contributes materially to its distinct- 

 ness, but is not, I think, properly called the cause of the bow. 

 The brilliancy not only of the bow, but also of the illuminated 

 space to some distance within it, is further increased by the 

 circumstance, that the drops in this situation return two sets 

 of rays to the eye, arising from light incident on both sides of 

 the angle, producing the maximum deviation. 



The only parts of the theory of the rainbow, which appear 

 to be generally understood, are the circumstances which deter- 

 mine the limits and arrangement of the coloured bands, and 

 the various angles at which the several colours arrive at their 

 maxima or minima * of deviation. It is not, I think, commonly 

 imagined that the circular streak of red is merely part of a cir- 

 cular red space, the interior of which is rendered undistinguish- 

 able by the superposition of other colours. That this is the 

 case may be made evident by observing the progress of the 

 rays through a sphere or cylinder of water, placed at various 

 angles, with a luminous body, and the eye. If it be desired to 



* Hitherto nothing has been said about a minimum of deviation ; but it will 

 presently be seen that the primary bow is formed by those rays which suffer a 

 maximum of deviation as compared with its illuminated space, while the secondary 

 bow is fonned by those rays which suffer a minimum deviation as compared with 

 iis illuminated space. Tins distinction must be borne in mind, because the rays 

 which, in the secondary bow, are. by comparison with the rest of the illuminated 

 space, said to be suH'eiing a 'minimum of deviation, do, in fact, deviate more than 

 those which, iu the primary bow, are said to be suffering a maximum. 



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