,\^v;.^ .3 q/* Light and its Theory. , ^" 111 



is confirmed bj the following experiment : I traced parallel 

 lines upon a plate of glass, covered with a thin layer of fat, in 

 such a way that the fat in every line must have been cut 

 sharp on one side, and less defined on the other, and actually 

 obtained through this warp an appearance similar to that of the 

 system before mentioned. If the ray does not fall vertically 

 on the system of lines, but inclines towards it in the plane which 

 vertically intersects the parallel lines, the effect is the same a9 

 if for these rays the distance of the parallel lines from each 

 other, that is g, were smaller in the proportion of the radius 

 to the cosine of the angle of incidence than by a light received 

 vertically. Consequently the distances of the spectra from 

 the axis (^) must become as much greater as the angle of in- 

 cidence is greater, because (as the equation II. shows) the se- 

 ries of these distances increase in the same proportion as s de- 

 creases. If, therefore, c denotes the angle of' incidence, that 

 is, the angle which the incident ray forms with a line perpen^ 

 dicular to the plane of the glass, it was safe to conclude, from 



T-r • c\(v) VU . . 0,(1/) VCt. 



equation II. viz. ^^ '' =r — , that sin. ^^ ^ = — ^ . 



^ S £. COS. tf 



But according to the theory of these phenomena, which 

 will be adverted to hereafter, it may be predicted, that in this 

 case the spectra on both sides of the axis will no longer be 

 symmetrical ; that, tTierefore, D', for instance, on the one side 

 of the axis must be larger than D^ on the other side. This 

 is also confirmed by experiments which will be mentioned 

 afterwards. In systems of lines in which s is not very small 

 the difference is not striking ; * but it is uncommonly great on 

 the contrary in those systems where ^ = 0,0001223 of an inch ; 

 for when <r = 55°, we have on one side of the axis D^ = 15° 6^, 

 and on the other side of the same axis D^ = 30* 33^ 



Now, if the symmetrical spectra of the first class, which do 

 not consist of homogeneous colours, and contain no fixed 

 lines, "I- are important for the theory of these phenomena, the 



* In. fact, with coarse systems of lines, when <r is not very large, one 



may use the term sine S- = with sufficient accuracy, as I have 



done in my Treatise " On the Neio Modification of Light " page 62; but 

 we shall farther on find an equally accurate and more simple term. 

 T New Modificaiion of Light, &c. page 12, French translation, p. 6S. 



