452 MossoTTi ON fraunhofer's reticular spectra. 



the colours would be symmetrically distributed on both sides ; 



and the lengths of the fits of the rays, which correspond to two 



colours equidistant from the centre of the spectrum, would pretty 



nearly satisfy the condition which was first noticed by Blanc, 



that its product was constant and equal to (i)! ; this leads to the 



equation 



-11 

 A <f> = 511-6(1) 3 'T, 



which gives the length of the fits Ai^, corresponding to the 

 arc 4" counted from the middle and assumed as positive towards 

 the red end, in millionths of a millimetre. 



The three series, (1.), (2.), (3.), which we have here combined, 

 comprise in a single point of view the simple relations by means 

 of which Newton ingeniously attempted to ascertain the different 

 elements of the spectrum. 



