MOSSOTTI ON FRAUNHOFEr's RETICULAR SPECTRA. 439 



These values correspond to the violet and red extremities of the 

 spectrum ; and as the second value is twice as great as the first, 

 it is evident that the length of the waves of the extreme red ray 

 amounts to twice that of the extreme violet, when these extremes 

 are observed (as was done by Fraunhofer) by means of a telescope, 

 and if we stop at that point -where the colours are still perfectly 

 distinguishable. 



If, in the same formula (1.), we make ?> = 0, we have 

 A„ = 553-5, 



i. €. in the centre of the spectrum the length of the waves 

 amounts to 553*5 millionths of a millimetre. Now we have 

 remarked that the centre corresponds to the maximum of the 

 intensity of the light ; supposing then that in every part of the 

 spectrum there exists an equal number of rays, we should say 

 that those, the waves of which have a length of 553*5 mil- 

 lionths of a millimetre, are most active in exciting in us the 

 perception of light, and that this capability of producing the phy- 

 siological effects of vision, both when the length of the waves 

 increases as well as diminishes, becomes lessened, and finally 

 almost vanishes, when the waves have increased or diminished 

 by one-third of the length corresponding to the maximum 

 effect. 



5. From the simplicity of these results, we conclude therefore 

 that, to ascertain the distribution and nature of the rays com- 

 posing solar light, it is of importance to make use of a spectrum 

 formed by means of a grating, as this alone is normal. In this 

 spectrum the light is symmetrically distributed from its centre, 

 and the relation between the length of the waves of the rays and 

 the distances from the centre in which their corresponding 

 colours appear in the spectrum, is by a simple law directly 

 given by experiment. 



The properties of the reticular spectra above detailed, and 

 the conclusion which I have deduced from them, — that they 

 yield new numerical data for optical questions, — appeared to me 

 sufficiently important to be communicated to this honourable and 

 learned assembly. 



Part II. — Analysis. 



The second part contains the mathematical proofs of the de- 

 duction announced in the first part. 



