50 OPTICS. 



colours which he saw were blue and 

 yellow. Whenever the colours of the 

 spectrum were absorbed by a reddish 

 glass, excepting red and dark green, he 

 saw only one colour, viz. yellow or 

 orange, which he could not distinguish. 

 When the middle of the red space was 

 absorbed by a blue glass, he saw the 

 black line, with what he called the yel- 

 low on each side of it. 



In these various cases the persons 

 are insensible to red light, and all the 

 colours into which it enters. Mr. Dalton 

 thinks it probable, that the red light is, 

 in these cases, absorbed by the vitreous 

 humour, which he supposes may have 

 a blue tint. If, which is probable, the 

 choroid coat be essential to vision, we 

 may ascribe the loss of red light, in cer- 

 tain eyes, to the retina itself having a 

 blue tint. If the dissection of the eye of 

 any person who possesses this peculia- 

 rity shall not establish any of these two 

 suppositions, we must content ourselves 

 with supposing that the retina is insen- 

 sible to the colours at one end of the 

 spectrum, just as the ear of certain per- 

 sons has been proved, by Dr. Wollaston, 

 to be insensible to sounds at one extre- 

 mity of the scale of musical notes, while 

 it is perfectly sensible to all other sounds. 



CHAPTER XVIII. 



Explanation of Natural Phenomena 

 I. Rainbow 2. Halos and Parhelia 

 3. Phenomena of the Mirage, or 

 Unusual Refraction 4. Colours of 

 Natural Bodies 5. Colours of the 

 A tmosphere 6 . Coloured Shadows 

 7. Converging and Diverging beams. 



1 . On the Rainbow. The rainbow 

 consists of two bows, or arches, ex- 

 tended across the part of the sky which 

 is opposite to the sun, and glowing with 

 all the colours of the prismatic spec- 

 trum. The principal rainbow, or the 

 innermost of the bows, which is most 

 commonly seen by itself, is part of a 

 circle whose diameter is 82, and is 

 nothing more than an infinite number 

 of prismatic spectra of the sun ar- 

 ranged in the circumference of a circle ; 

 the colours being the very same, and 

 occupying the same space as in the 

 spectrum produced from the sun's light, 

 The red rays form the 'outermost portion, 

 and the violet rays the innermost portion 

 of the bow. The secondary, or external 

 bow, is much fainter than the other, and 

 has the violet outermost, and the red 

 innermost : it is part of a circle 1 04 in 

 diameter. 



As this interesting phenomenon is 

 never seen unless when the sun is shin- 

 ing, and when rain is falling between 

 the spectator and the part of the horizon 

 where the bow is seen, it has been uni- 

 versally ascribed to the decomposition 

 of the white light of the sun by the re- 

 fraction of the drops of rain, and their 

 subsequent reflexion within the drops ; 

 and this supposition is sufficiently pro ved 

 by the fact, that rainbows are produced 

 by the spray of waterfalls, and may be 

 made artificially, by scattering water 

 with a brush or syringe when the sun 

 is shining. 



In order to explain the production of 

 the rainbow, let us suppose that the 

 observer, placed at E,/7g\ 48, is looking 

 through a shower of rain at the part of 

 the sky opposite to the sun when he is free 

 from clouds. Let A be a drop of rain, 

 and S R a ray of the sun falling upon the 

 upper side of it, A 11. Those rays which 

 pass through the middle of the drop will 

 fall upon it, and form an image of the 

 sun in the focus of the drop, as explained 

 in Chap. iii. p. 9, and therefore we con- 

 sider only those which fall obliquely on 

 the drop. Some of the rays of the 

 beam S 11 will suffer reflexion at R, 

 but the greater number will enter the 

 drop, and suffer refraction. The violet 

 light of the beam will be refracted in 

 the direction It v, and the reel in the 

 direction It r, all the intermediate co- 

 lours lying between these two. Some 

 of these rays will pass out of the drop 

 at v and r, being refracted a second 

 time ; but none of them can reach the 

 eye at E. Those, however, which suffer 

 reflexion at v r will return through the 

 drop, the red ray R r in the direction 

 r r', and the violet ray It v in the di- 

 rection v v', and experiencing a second 

 refraction at the points r 1 v', they will 

 issue from the drop, and proceed to the 

 eye of the observer at E ; who will then 

 see all the prismatic colours between r' E 

 and v' E projected on the opposite sky. 

 Those drops of rain which are directly 

 between the observer E, and the point of 

 the sky opposite to the sun, will form 

 the upper part of the coloured arch ; 

 those drops which are to the right hand 

 of the observer, and near the ground, 

 will form the right-hand extremity of 

 the bow ; and those to the left hand of 

 the observer, and near the ground, will 

 form the left-hand extremity oi the bow. 

 Drops having an intermediate position, 

 and an intermediate height, will form 

 the intermediate parts of the low. If 



