200 Dr. Forster on the Variation of the reflective, 



paper, and have never seen his observations, I cannot speak 

 positively to that point. 



If the opinion which I have long entertained, that the mean 

 refractive power of the atmosphere varies in different places, 

 be correct ; its promulgation may lead to important results : 

 it may explain certain apparent discordances in the places 

 assigned to the fixed stars as described in different catalogues. 

 For if corrective tables calculated for Berlin or Paris were 

 to be applied to observations at Greenwich, there might be a 

 considerable difference in the result. 



Another very important observation results from this con- 

 sideration. Suppose, for example, a given corrective table 

 of mean refraction were to be applied to a catalogue of ap- 

 parent places of the fixed stars observed at Pisa a hundred years 

 ago, and that the same table were to be applied to a catalogue 

 observed this year at Dublin, the result of a comparison of the 

 two catalogues would be a positive error. For Dublin and 

 Pisa requiring a different correction, altogether independent 

 of longitude and latitude, and depending solely on the dif- 

 ferent refractive powers of their respective atmospheres, the 

 consequence of applying the same corrective table to the ob- 

 servations of both places would be the certain error of one of 

 the catalogues. And as one catalogue would, according to our 

 supposition, be a hundred years older than the other, the stars 

 would be thought to have changed their places. Some misap- 

 plication of tables of refraction seems to me to be the cause of 

 an idea of a southern motion recently promulgated by certain 

 astronomers. Independently of the great prima facie improba- 

 bility of such a motion, it seemed to me, when I first heard the 

 opinion mentioned, that it would turn out to be an error de- 

 pending on the want of a proper application of corrective tables 

 of refraction. I speak, however, on the point with deference 

 to the better judgement of learned astronomers. 



III. 



Of Varieties in the Composition and Nature of the Light of 

 different Stars, considered as still further varying the Effects 

 of atmospherical Refraction, Reflection and Dispersion. 



It must have occurred to almost every body to perceive 

 that the colour of different stars is very dissimilar ; that some 

 appear more red or copper coloured, others yellow like brass ; 

 some of a bright and almost silvery whiteness, while yet others 

 are of a dull white colour. These differences become much 

 more striking when viewed through such bad telescopes as, 

 being but imperfectly achromatic, become in a measure pris- 

 matic, and separate the primitive rays of light, producing a 



coloured 



