J^iv^tCAL OPTICS, ^ I'Sl 



Ifernal fringes, there are two or three alter nations'of colours, 

 beginning from the line which bifefls the angle, difpofed on 

 each fide of it, in curves, which are convex towards the bi- 

 fedting line, and which converge in fome degree towards it, as 

 they become more remote from the angular point. Thefe 

 fringes are alfo the joint efFed of the light which is inflefled 

 diredlly towards the fliadow, from each of the two outlines of 

 the object. For, if a fcreen be placed within a few inches of 

 the objedt, fo as to receive only one of the edges of the fliadbw, 

 the whole of the fringes difappear. If, on the contrary, the 

 redlangular poinf of the fcreen be oppofed to the point of the 

 fliadow, fo as barely to receive the angle of the ftiadow on 

 its extremity, the fringes will remain undifturbed. 



ir. COMPARISON OF MEASURES, DEDUCED FROM VARIOUS 

 EXPERIMENTS, 



If we now proceed to examine the dimenfions of the fringes, Comparifoii ot 

 under different circumftances, we may calculate the differences incaf"^^' 

 of the lengths of the paths defcribed by the portions of light, 

 which have thus been proved to be concerned in producing 

 thofe fringes ; and we fliall find, that where the lengths are 

 equal, the light always remains white; but that, where either 

 the brighteft light, or the light of any given colour, difappears 

 and reappears, a firft, a fecond, or a third time, the differences 

 of the lengths of the paths of the two portions are in arithme- 

 tical progreflion, as nearly as we can exped experiments of this 

 kind to agree with each other. I ftiall compare, in this point of 

 view, the meafures deduced from feveral experiments of New- 

 ton, and from fome of my own. 



In the eighth and nmth obfervations of the third book of 

 Newton's Optics, fome experiments are related, which, toge- 

 ther with the third obfervation, will furnifti us with the data 

 necetfary for the calculation. Two knives were placed, with 

 their edges meeting at a very acute angle, in a beam of the fun's 

 light, admitted through a fmall aperture ; and the point of con- 

 courfe of the two firft dark fines bordering the fhadows of the 

 refpedive knives, wasobferved at various difiances. Therefults 

 of fix obfervations are expreffed in the firfl three lines of the 

 firfl Table. On the fuppofition that the dark line is produced 

 by the firft interference of the light refledted from the edges of 

 the knives, with thelight pafling in a ftraight line between them, 



K2 we 



