Royal Society of London. 363 



of it, that shines upon us from any one, even the most brilliant, of the 

 fixed stars. 



" It may be remembered, that on a former occasion, in examining 

 the performance of a good telescope, I found that the sun's image, 

 reflected from the surface of a small sphere, (such as that of a ther- 

 mometer bulb, filled with mercury,) and viewed at a proper distance 

 through a telescope, is, to all appearance, extremely like a fixed star, 

 and forms in such experiments, an admirable substitute for one, in 

 being really fixed, and therefore well adapted for deliberate observa- 

 tion. It occurred to me, while engaged in this examination, that by 

 comparing such an image with one of the larger stars, I might be able 

 to obtain some grounds for estimating the fight of the star. 



" It would be desirable, though extremely difficult, in conducting such 

 an experiment, to make a direct comparison between the star and the 

 sun's image ; since in that case we should be enabled to avoid the 

 uncertainties arising from an indirect comparison, the consequence of 

 observing at times so distant, that the atmosphere has in the interval 

 undergone considerable changes. As, however, the only practicable 

 mode of observing is the indirect one, by comparing the two objects 

 with some common standard at diflerent times, we must endeavor to 

 remove these uncertainties from our results, by repeating each series ^ 

 of comparisons so frequently, that the average of each series may be 

 afiected by atmospheric vicissitudes, or may fairly be presumed to be 

 so, in an equal degree. 



" The common standard of comparison I chose, was the image of 

 a candle, reflected from a small thermometer bulb, (in most trials, 

 about one quarter of an inch in diameter) filled with mercury, and 

 seen by one eye through a lens of about two inches focus, at tlie 

 same time that the sun's image, (reflected in a similar manner,) from 

 a thermometer bulb, placed at a distance, was viewed by the other 

 eye through a telescope." 



The precautions used, and the detail of the observations, would 

 exceed our limits ; we shall therefore pass to the results. These 

 are : that the light of the brightest of the stars, Sirius, does not ex- 

 ceed a 20,000,000,000th part of the sun's light ; and tliat of Lyra 

 is about the 180,000,000,000th part of the same, or one-ninth part 

 of the light that reaches us from Sirius. 



The fourth paper of WoUaston is stated by him, to be communi- 

 cated for the purpose of doing justice to the memory of Dr. Marcet, 

 by recording one of his latest efforts in the cause of science. It is 

 well known that a current, at the surface, is continually setting 



