28 



i-Vth of an inch in length ; but there was no appearance of ignition, 

 although the same wire may be instantly ignited by a single pair of 

 one inch plates immersed in a weak acid. 



On the dispersive Power of the Atmosphere, and its Effect on astrono- 

 mical Observations. By Stephen Lee, Clerk and Librarian to the 

 Royal Society . Communicated by W. H. Wollaston, M.D. Sec. R.S. 

 Read June 15, 1815. [Phil. Trans. 1815,^. 375.] 



Although the appearances of colour given to low stars by atmo- 

 spheric refraction be very well known, the comparative degree of re- 

 frangibility of the differently coloured rays does not appear to have 

 attracted attention in proportion to its important effects on delicate 

 astronomical observations. 



The author endeavours to point out some of the principal errors 

 that may arise from making allowance for mean refraction without 

 due discrimination of the kinds of colour observed. It is evident that 

 stars of different colours will require different corrections in observa- 

 tions of their altitudes. 



The apparent altitude of the sun will also vary, according to the 

 coloured glass employed in viewing its disc. For since there must, 

 in fact, be several images of the limb observed at small distances from 

 each other, it becomes a matter of choice which of them shall be 

 selected by the kind of glass used ; and it is possible, that to their 

 cause may be ascribed the discordance which exists between the ob- 

 servations of the solstices, and possibly some disagreement between 

 different observers. 



Mr. Lee also suggests, from this source, an explanation of the 

 apparent projection of Aldebaran and other red stars upon the sur- 

 face of the moon, a phenomenon that has been frequently noticed, 

 but not yet understood. For if Aldebaran be nearly in contact with 

 the upper limb, since the white light of the moon will be elevated 

 more by refraction, it is evident that the stars may thus be made to 

 appear within her disc a few seconds before or after contact. 



The author refers to a great number of observations that he has 

 made on Mars, Venus, and fixed stars ; from which he infers the 

 quantity of dispersion of light to be between one sixtieth and one 

 seventieth of the total atmospheric refraction. 



He also adds several remarks on certain alterations in the mode of 

 making astronomical observations, by which the results deduced may 

 have been affected, especially with reference to those of Dr. Bradley ; 

 noticing particularly the period when Hadley's sextant came into 

 general use, and with it the employment of glasses variously coloured, 

 which were soon applied to other instruments. 



Mr. Lee concludes with suggesting such precautions as may lead 

 to a more correct knowledge of atmospheric refraction, hoping that 

 the subject may be pursued by astronomers more favourably situated 

 than himself for such an investigation. 



