112 M. Fraunhofer on the Laws of Light. 



nofi symmetrical spectra of the second class are far more in- 

 teresting, because they show the fixed lines ; and from them 

 the law of this modification of light can be deduced with ex- 

 treme accuracy, and the theory of these phenomena can be 

 put to the severest test. I give here the result of several 

 experiments with the coloured rays D and F. In order to 

 show on which side of the axis the angle S lies, which measures 

 the distance of the coloured ray from the axis, I denote with 

 —I, — II... those spectra which lie on that side of the axis, where 

 the received ray inclines to the plane of the warp ; and on 

 the other hand, with + i, + ii...the spectra at the opposite 

 side of the axis, where the obliquely incident ray makes ob- 

 tuse angles with the system of lines, so that D— ^ D +i, or 

 D— ", D +11... denote the opposite situations of the rays D in 

 the first or second spectrum. The angle of incidence c is here 

 to be understood in the same sense as above, that is, the angle 

 which the incident ray forms with the perpendicular. 



Experiments with the system of lines on glass in which 

 i = 0,0001223. 



ff=z55° 



