THEORY OF LIGHT AND COLOURS. >-j7 



drawn on glafs, at the diftance of one five hundredth of an cTftance of one 

 inch, are the moft convenient. Each of thefe lines appears fiw hundredth 

 under a microfcope to confifr of two or more liner lines, ex- 

 actly parallel, and at the diftance of fomewhat more than a 

 twentieth of that of the adjacent lines. I placed one of thefe when theangu- 

 fo as to reflect the fun's light at an -angle of 45°, and fixed it l*r poliiion of 



- r , , • . • , , r i i- thefe 1S variC( l 



in luch a manner, tnat while it revolved round one ot the lines t h ey produce 

 as an axis, I could meafure its angular motion ; and I found, colours, 

 that the brighteft red colour occurred at the inclinations 10^°, 

 20^°, 32°, and 45 y ; of which the fines are as the numbers 

 ], 2, 3, and +. At all other angles alio, when the fun's light 

 was reflected from the furface, the colour vanifhed with the 

 inclination, and was equal at equal inclinations on either fide. 



This experiment affords a very ftrong confirmation of the which ftrongly 

 theory. It is impoffible to deduce any explanation of it from *^ fi ^ t u £ vdy 

 any hypothecs hitherto advanced; and I believe it would be theory. 

 difficult to invent any other that would account for it. There 

 is a ftriking analogy between this feparation of colours, and 

 the production of a mufical note by fucceflive echoes from it is analogous 

 equidiftant iron naliiades : which I have found to correspond *> the mufical 



..... . r r i it ec h° fr° m lr on 



pretty accurately with the known velocity ot iound, and the ra ii 8% 

 diftances of the furfaces. 



It is not improbable that the colours of the integuments of Verfatile colours 

 tome infects, and of fome other natural bodies, exhibiting in of fome Mtf*&*» 

 different lights the moft beautiful verfatility, may be found to tn i s ' nature. 

 be of this defcription, and not to be derived from thin plates. 

 In fome cafes, a tingle fcratch or farrow may produce fimilar 

 effects, by the reflection of its oppofite edges, 



Corollary 2. Of the Colours of thin Plates. 



When a beam of light falls on two parallel refracting fur- Colours of thin 

 faces, the partial reflections coincide perfectly in direction ; P !ates explained 

 and, in this cafe, the interval of retardation, taken between 

 the furfaces, is to their diftance as twice the cofine of the angle 

 of refraction to the radius. For, in Fig. 3, Plate VI. draw- 

 ing A JB and C D perpendicular to the rays, the times of paf- 

 fing through B C and A D will be equal, and D E will be hair" 

 the interval of retardation ; but D E is to C E as the fine of 

 DC E to the radius. Hence, that D E may be conusant, or by the greater 

 that the fa:ne colour may be reflected, the thicknefs C E mnft length of ray re- 

 vary as the iecant of the angle of refraction Q E D : which pofteriorfarface! 



agrees which muft pro* 

 ° duce an effect 



