DIFFRACTION. 123 



terval of the axes of the wires, may be ascertained with the 

 greatest ease and precision ; and we have, therefore, only to 

 measure the angular deviation, 0, of the ray of any simple 

 colour from the axis, in order to deduce the value of A. 

 Fraunhofer computed in this manner the lengths of the 

 waves corresponding to the seven principal fixed lines of the 

 spectrum ; and the resulting values are perhaps the most 

 exact optical constants we possess. 



The wave-lengths, corresponding to the principal fixed 

 lines, B, C, D, E, F, Or, H, expressed in millionths of a 

 millimetre, were thus found to be 



656, 589, 526, 484, 429, 393. 



The wave-length corresponding to the middle point of the 

 diffracted spectrum is 553' 5 millionths of a millimetre. The 

 wave-lengths corresponding to the extreme visible points are 

 738 and 369 millionths, respectively, the former of which is 

 exactly double of the latter. 



(136) But the diffracted spectrum is further distinguished 

 by the simplicity of the law which governs the intensity of the 

 light in its several parts. The intensity of the light in the or- 

 dinary spectrum (formed by a prism of flint-glass) was deter- 

 mined by Fraunhofer, for the points corresponding to the 

 principal fixed lines. These intensities are represented by the 

 ordinates of the curve in the annexed diagram. The ordinate 



B c 

 at the point m, situated between the fixed lines D and E, at a 



rj 



distance Dm = DE, corresponds to the maximum inten- 

 sity, and divides the whole light of the spectrum into two 



