and the Absorptio7i of the Tithonic Rays. 467 



As respects its tithonic effects the same difficulty occurs, for 

 these are necessarily controlled and disturbed by the law of 

 distribution. All chemical actions occurring in the more re- 

 frangible regions, by being spread over a great space, appear 

 to be more feeble than what they actually are. 



In a perfect spectrum the most luminous portion of the 

 yellow should be in the centre; and from this the intensity of 

 the light should gradually decline, fading away on one side in 

 the red, and on the other in the violet. At equal distances 

 frotn the middle yellow point, on either hand, the intensity of 

 the light should be equal. These beautiful results are due to 

 Mossotti, who also shows that the length of the extreme red 

 wave, is to that of the extreme violet, in the simple propor- 

 tion of 2 : 1 . 



The prismatic spectrum does not exhibit these facts. The 

 yellow is not in the centre; the blues are abnormally spread 

 out, the spectrum having its own law of distribution. But the 

 interference spectrum enables us to observe them. Its pho- 

 tographic action on the retina accords with the above-men- 

 tioned conditions; but its tithonographic action, as I shall 

 presently show, appears not to correspond thereto. 



By the aid of a heliostat, I arranged horizontally in a dark 

 room a narrow riband of light, coming through a fissure ^oth 

 of an inch wide. At the distance of twelve feet it fell per- 

 pendicularly on a piece of flat glass, the surface of which was 

 ruled with equidistant parallel lines by a diamond, and having 

 been silvered with tin foil, after the manner of a mirror, it served 

 the purpose of a grating. The reflected beam went out through 

 the aperture at which it entered ; and on either side of it, to 

 the right and left, the well-known double series of interference 

 spectra made their appearance. I selected, for the obvious 

 reason that it was not overlapped by its successor, the first of 

 one of the series, and intercef)ting it by an achromatic object- 

 glass, placed in the focus a frame capable of holding a ground 

 glass or sensitive surfaces. This frame was adjusted until the 

 fixed lines were distinctly depicted upon it. 



For a further description of the reflected interference spec- 

 trum, I may refer to any of the elementary works on optics. 

 It is sufficient for my purpose here that the reader should bear 

 in mind, that the angular deviations of any two colours from 

 the primitive incident ray are to one another as the lengths of 

 their respective undulations. 



But on the ground glass we see the fixed lines, and the 

 length of waves corresponding to those lines has been rigo- 

 rously determined by Fraunhofer. The following Table is 

 extracted from Sir J. Herschel's treatise on Light ; the Paris 



2 12 



