on passing through the Atmosphere. 273 



although it ought to be presumed that the solar spectrum 

 should undergo variations flowing from this action; as 

 no astronomer, to my knowledge, has announced, that it! 

 the experiments which have been repeated at various times, 

 and in different places, variations have heen observed in the 

 colour of the spectrum, and as all are silent as to a phseno- 

 menon so singular ; it might be supposed that the cause of 

 the colouring of the disk depended on an order of alteration 

 with which the spectrum was not affected. 



Those hypotheses which attribute to the molecules of air 

 properties so different were presented under an aspect more 

 or less seducing; all of them could be discussed, defended, 

 and adopted, if the reasonings and authorities of authors 

 had been sufficient : but as none of those who have proposed 

 them have supported them by any positive facts, I appeal 

 to experience. 



The most natural experiment, and that which every author 

 ought to make before proposing his hypothesis, is the ana- 

 lysis of the rays of light when the disk of the sun is present- 

 ed under colours so various. This analysis has been effected, 

 and I now present the results to the class. 



I fixed upon some fine days in the summer of 1799, when 

 the sky was pure and the disk of the sun white, which al- 

 ways happens towards noon, when the sun is at its greatest 

 elevation. 



With this view T introduced a solar ray into a dark room 

 through an aperture of the size of 25 decimillimeti^s : I 

 received it on the surface of a prism, which I turned so as 

 to make the angles formed by the ray refracted, and the 

 two faces of the prism equal. I observed at one and the 

 same time both the series of the colours of the spectrum 

 and its length at 30 decimetres distance from the prism. 

 I remarked that all the colours were perfectly distinguished 

 from the purple to the red, and that on the evening of the 

 same day, at sunset, when its disk appeared yellow, the 

 spectrums formed by the solar rays were not so long : the 

 purple no longer existed, and a greater or less portion of 

 the violet was wanting, and sometimes it was even entirely 

 wanting. 



The experiments thusmade at noon and evening were re* 

 commenced m the fine weather of the following years, and 

 gave the same result. 



I have the honour to present to the class three spectrums 

 obtained with the same prism on the 13th of January 1801. 

 They are remarkable for their length, and for the colours 

 which i'ovm them. At half past ten o'clock A.M., the 

 spcclrum, fig. 1, (Plate VII.) was 145 millimetre* long; 



Vol. 36. No. 150. Oct. J 810. S at 



