TFlOF THE SUN WITH THAT OF THE FIXED STARS. ^tl 21 



the sun is, than that exceedingly minute quantity of it which 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 correct 

 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 thermometer-bulb filled with mer- 

 cury,) and viewed at a proper distance through a telescope, is, to appearance, 

 extremely like a fixed star, and forms, in such experiments, an admirable sub- 

 stitute for one, in being really fixed, and therefore well adapted for deliberate 

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

 comparing such a reflected image with one of the larger stars, I might be 

 able to obtain some grounds for estimating the light of the star. 



It would be desireable, 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 inse- 

 parable from an indirect comparison, the consequence of obsei'ving at times 

 so distant, that the atmosphere in the interval has undergone considerable 

 change. As, however, the only practicable method of observing is the indirect 

 one, by comparing the two objects with some common standard at different 

 times, we must endeavour to remove those uncertainties from our results, by 

 repeating each series of comparisons so frequently, that the average of each 

 series may be affected by atmospheric vicissitudes, or may fairly be presumed 

 to be so, in an equal degree. 



The common standard of comparison which I chose, was the image of a 

 candle, reflected from a small thermometer-bulb, (in most trials about ^th o{ 

 an inch in diameter,) filled with mercury, and seen by one eye through a lens 

 of about two inches focus, at the same time that the sun's image, reflected (in 

 the manner above described) from a thermometer-bulb placed at a distance, or 

 the star itself, was viewed by the other eye through a telescope. 



In order to make the light of the two objects, when seen through the tele- 

 scope, and that of the candle, more nearly alike in colour, I placed two yellow 

 glasses at the eye-piece ; and I thought it expedient to have in view, at the 

 same time with the subject of comparison, two candles, one of tallow, the 

 other of wax ; that by making the star, or the little sun, a mean between the 



i;ij. JUJ U.I (U r 



