J70 THEORY OF LIGHT AND COLOURS. 



fite to produce colours, does by degrees grow to the greateft 

 thicknefs, the colours (hall be fo often repeated, as the weaker 

 pulfe does lofe paces with its primary or firft pulfe, and is co- 

 incident with a" fubfequent ** pull!'. And this, as it is coin- 

 cident, or follows from the firft hypothecs. I took of colours, 

 fo upon experiment have I found it in multitudes of inftances 

 which wa» that feem to prove it." (P. 65 — 67.) This was printed 

 vewTbefav* about feven years before any of Newton's experiments were 

 Newton made made. We are informed by Newton, that Hooke was after- 

 hi« experiments. wards Jifpofed to adopt his " fuggeftion" of the nature of 

 colours ; and yet it does not appear that Hooke ever applied 

 that improvement to his explanation of thefe phenomena, or 

 inquired into the neceflary confequence of a change of obli- 

 quity, upon his original fuppofition, otherwife he could not 

 but have difcovered a flriking coincidence with the meafures 

 laid down by Newton from experiment. AH former attempts 

 to explain the colours of thin plates, have either proceeded 

 on fuppofitions which, like Newton's, would lead us to ex- 

 pect the greateft irregularities in the direction of the refracted 

 rays ; or, like Mr. Michell's, would require fuch effects from 

 the change of the angle of incidence, as are contrary to the 

 effects obferved ; or they are equally deficient with refpect to 

 both thefe circum fiances, and are inconfifient with the moil 

 moderate attention to the principal phenomena. 



Corollary 3. Of the Colours of thick Plates. 



Colours of thick When a beam of light paflfes through a refracting furface, 

 plates explained efpecially if imperfectly polifhed, a portion of it is irregularly 

 length of fuch Mattered, and makes the furface vifible in all directions, but 

 rays of fcatceredmoft confpicuoufly in directions not far diftant from that of the 

 Sthrprinct 1 ^^^ 1 ^ and > ^reflecting &** be placed parallel to 

 pal ray) as pafs the refracting furface, this fcattered light, as well as the prin- 



through the firft ci j be wi j| be re fl e &ed, and there will alfo be a new dif- 



furface, and are r , ' 



reflected from npation of light, at the return of the beam through the refract- 



the fecondwith j n g f ur f a ce. Thefe two portions of fcattered light will coin- 

 cide in direction ; and, if the furfaces be of fuch a form as to 

 collect the iimilar effects, will exhibit rings of colours. The 

 interval of retardation is here, the difference between the 

 paths of the principal beam and of the fcattered light between 

 the two furfaces ; of courfe, wherever the inclination of the 

 fcattered light is equal to that of the beam,, although in diffe- 

 2 rent 



