166 THEORY OF LIGHT AND COLOURS. 



ticed them. Mazeas and Mr. Brougham have made fome ex- 

 periments on thefubject, yet without deriving any fatisfactory 

 conclufion. But all the varieties of thefe colours are very 

 caiily deduced from this propofition. 

 by the greater Let there be in a given plane two reflecting points very near 



(oHiw rf"n- each other ' and let the P lane be fo fixated that the refleded 

 duiation) from image of a luminous object feen in it may appear to coincide 



the deprefled with t{)e j ntg then ^ fe obv ; ous (h t t j j th f fa { • 



rortion of (ur- j 



ace, dent and reflected ray, taken together, is equal with reipect 



to both points, confidering them as capable of reflecting in all 

 directions. Let one of the points be now deprefled below the 

 given plane; then the whole path of the light reflected from 

 it, will be lengthened by a line which is to the depreflion of 

 the point as twice the cofine of incidence to the radius. Fig. 2. 

 Plate VI. 

 which will occa- If, therefore, equal undulations of given dimensions be re- 

 oHnttnCitY anT ^ e ^ e< ^ fr° m two points, fituated near enough to appear to 

 remiflion in the the eye but as one, wherever this line is equal to half the 

 compound undu- breadth f h j undulation, the refledion from the de- 

 lation ; accord- . .-'/.' . 



ingly as the de- preffed point will fo interfere with the reflection from the 

 preffion caufes fi xe d p 0U1 t that the progreflive motion of the one will coin- 



the undu ations . , . V , i • r ^ i . . -,i 



of the fucceed- C] de with the retrograde motion or the other, and they will 

 ing ray to coin- both be deftroyed ; but, when this line is equal to the whole 

 pofe thofe of the Dre adth of an undulation, the effect will be doubled; and 

 preceding ray. when to a breadth and a half, again deftroyed ; and thus for 

 a confiderable number of alternations : and, if the reflected 

 undulations be of different kinds, they will be varioufly af- 

 fected, according to their proportions to the various length of 

 the line which is the difference between the lengths of their 

 two paths, and which may be denominated the interval of 

 retardation, 

 Experiment by In order that the effect may be the more perceptible, a 

 parallel ftrokes j number f p a j rs f points muil be united into two parallel lines ; 

 and, if feveral fuch pairs of lines be placed near each other, 

 they will facilitate the obfervation. If one of the lines be 

 made to revolve round the other as an axis, the depreflion 

 below the given plane will be as the fine of the inclination; 

 and, while the eye and luminous object remain fixed, the 

 difference of the length of the paths will vary as this fine, 

 fuchas the fine The belt fubjects for the experiment are -Mr. Coventry's 

 micrometers of eX q U ifit e micrometers ; fuch of them as coniift of parallel lines 



drawn 



