332 Dr. Forster on the dispersive Power 



column, which is the most important, exhibits what I con- 

 ceive to be the proportional retraction of the different stars 

 severally, whose names are put down in the first column ; and 

 this scale of comparative refraction is founded on the position 

 laid down, and which I believe repeated experiment will con- 

 firm, — that stars are more or less refracted according to the 

 peculiar composition of their light: that is to say, in other 

 words, the relative quantity of refraction of each star se- 

 verally varies as the relative proportion of the more to the 

 less refrangible rays of which their mixed light is composed. 

 According to this position, it would seem that stars which 

 appear to possess the red or less refractive rays would require a 

 less, while those with a great proportion of violet would re- 

 quire a greater correction than the mean refraction as stated 

 in ordinary tables. 



I offer the following table with great deference to the re- 

 sults of other observations, and likewise with great caution, it 

 being founded on an attempted measurement of the propor- 

 tionate deviation and relative quantity of the differently co- 

 loured rays in the several stars stated in the table. And 

 I request that allowance be made for the great difficulty 

 of such observations ; and that the novelty of the experi- 

 ments, the few opportunities that I have had of repeating 

 them on an extensive scale, together with the great nicety re- 

 quisite where small quantities are concerned, — may apologize 

 for the rude and imperfect manner in which the observations 

 have been conducted. 



If the future observations of more able persons should con- 

 firm my own, I shall be glad to see them in detail: if they 

 should be confuted, the very detection of the cause of the 

 error may lead on to the knowledge of some other fact of use 

 in the very curious investigations now going forward as to the 

 nature of light. At all events they have afforded me, and 

 may afford to others, agreeable amusement during the long 

 and dreary winter nights of this miserable country. 



In the following table I assume the apparent altitude of the 

 stars above the horizon at the time of observation to be 10°, 

 and the barometer O m, 760: — the centigr. thermometer -f 10. 

 Under which circumstances we may estimate the mean re- 

 fraction as 5' 19"* 10, according to the formulae determined by 

 Laplace, and published in the Connaissance des Temps. Now 

 presuming this to be the real mean refraction, and believing 

 also that it equals the refraction of Capella, the figures put down 

 in column 3 of the following table, indicate the quantities of 

 refraction to be added to or subtracted from the above-men- 

 tioned mean, in order to get the true refraction of each of the 



stars 



