in the Solar Spectrum. 503 



dimensions of this and its distance from the screen. The 

 whole of these circumstances necessarily leads me to admit, 

 that these appearances of the luminous spectrum are produced 

 by the different distribution of the rays, caused by the diffei'ent 

 media which they have to pass through. The monochromatic 

 light of the lateral spectra, the production of such different 

 lines by the simple variation of the distance of projection, the 

 transformation of the systems of lines into forms so varied by 

 the alteration of size of the aperture through which the light 

 is admitted, the increase of tints in the obscure lines, and the 

 strengthening of the luminous ones, are, in my opinion, so 

 many clear proofs of variations in the reflexion, refraction, and 

 dispersion of light. I do not thereby wish to deny that any 

 ray is extinguished by absorption or by interference, but only 

 to show that these two acts, such as they are considered in the 

 undulatory system, are not causes really sufficient to produce 

 the phaenomena of the luminous spectrum. It cannot be ab- 

 sorption; for if the black lines are really produced by absoi'bed 

 rays, and not by rays differently distributed, it is impossible that 

 the increase or decrease of the black tints should always be 

 accompanied by an analogous variation in the intensity of the 

 luminous lines, and the thickening or attenuation of the black 

 lines always by the enlargement or diminution of the lumi- 

 nous lines. The large systems of obscure lines produced by 

 nitrous gas and by the vapours of iodine, are traversed in their 

 centre by a zone of very bright light three or four millimetres 

 wide. The systems of uniform and excessively fine lines pre- 

 sent a tranquil light, which is spread equally throughout the 

 height of the spectrum, so as not to be remarked by an eye 

 not accustomed to similar researches. It cannot be interfe- 

 rence ; for in the space or in the line itself, where rays which 

 should go on increasing the intensity of light should meet, 

 this still shows itself diminished or extinguished : the lines are 

 sometimes luminous, white, sometimes coloured, sometimes of 

 a more or less perfect black. I have verified this phaenomenon 

 some hundreds of times. It entirely upsets and destroys the 

 doctrine of interference, as I shall show in a future communi- 

 cation, in which I intend to make known the rest of my ob- 

 servations. 



" I said that the solar spectrum is the most exquisite 

 photoscope known. The following are some of the experi- 

 ments in support of this assertion. Before the aperture, 

 which was two millimetres wide, I placed a bottle of the 

 purest white crystal with parallel sides. No line. appeared in 

 the solar spectrum on the screen placed at 2"39 metres from 

 the prism. The bottle was at 0'"'1 from that, and the projec- 



