<)26' 



-ON THE THEORY OF LIGHT AND COLOURS. 



Mr. Co-rentry's exquisite micrometers; such 

 •of tliem as consist of parallel lines drawn on 

 glass, at the distance of one five hundreth 

 t)f an inch, are the most convenient. 

 Each of these lines appears under a micro- 

 scope to consist of two or more finer lines, 

 exactly parallel, and at the distance of some- 

 what more than a twentieth of that of the 

 adjacent lines. I placed one of these so as 

 to reflect the sun's light at an angle of 45°, 

 and fixed it in such a manner, that while it 

 revolved round one of the lines as an axis, I 

 could measure its angular mtKion ; and I 

 found, that the brightest red colour occured 

 at tlie inclinations 10^:°, 20^°, 32°, and 45°; 

 of which the ^ihes are as the numbers 1, 2, 3, 

 and 4. At all other angles also, when the 

 sun's light was reilcclcd from the surface, the 

 colour vanished with the inclination^' and 

 was equal at equal inclinations on either side. 



This experiment affords a very strong con- 

 firmation of the theory. It is impossible to 

 deduce any exi)lanation of it from any hy- 

 pothesis hitherto advanced ; and I believe it 

 would be difficult to invent any other that 

 would account for it. There is a striking 

 analogy between this separation of colours, 

 and the production of a nmsieal note by suc- 

 cessive echos from equidistant iron palisades; 

 which I have found to correspond pretty ac- 

 curately with the known velocity of sound, 

 and the distances of the surfaces. 



It is not improbable that the colours of the 

 integuments of some insects, and of some 

 other natural bodies, exhibiting in different 

 lights the most beautiful versatility, may be 

 found to be of this description, and not to be 

 derived from thin plates. In some cases, a 

 single scratch or furrow may produce similar 

 effects, by the reflections of its opposite edges. 



ConoLtARY n. Of the G&lours of thin 

 Platen. 



When abeam of light falls on two parallel 

 refracting surfaces, the partial reflections 

 coincide perfectly in direction ; and, in this 

 case, the interval of retardation, taken' between 

 tlie surfaces, is to. their distance as twice the 

 cosine of the angle of refraction to the raJ- 

 dius. For, in Plate 14. Fig. 107, drawing AB 

 and CD perpendicular to the rays, the times 

 of passing through BC and AD will be equal, 

 and DE will be half the interval of retarda- 

 tion ; but DE is to CE as the sine of DCE to 

 the radius. Hence, in order that DE rnay 

 be constant, or that the same colour may be 

 reflected, the thickness CE must vary as the 

 secant of the angle of refraction CED : 

 which agrees exactly with ISewton's experi- 

 ments; for the correction which he has intro- 

 duced is perfectly inconsiderable. 



Let the medium between the surfaces be, 

 riirer than the surrounding mediums ; then 

 the impulse reflected at the second surface, 

 meeting a subsequent undulation at the first, 

 will render the particles of the rarer medium 

 capable of wholly slopping the motion of the 

 denser, and destroying the reflection, (Prop. 

 4.) while they themselves will be more 

 strongly propelled than if they had been at 

 rest; and the transmitted light will be in- 

 creased. So that the colours by reflection 

 will be destroyed, and those by transmission 

 rendered more vivid, when the double thick- 

 nesses, or intervals of retardation, are any 

 multiples of the whole breadths of the undu- 

 lations; and at intermediate thicknesses the 

 eflects will be reversed : according to the 

 Newtonian observations. 



If the same proportions be found to hold 

 good with respect to thin plates of a denser 



