ON THE HORIZONTAL MOON. J57 



Rays from a diftant object which pafs through a convex lens Optical theorem, 



near its center, will have their focus more diftant from the Jr^ifftorcer* 



lens, than thofe rays which pafs through it nearer to its cir- the larger the 



cumference : and, confequently, the image formed by the a P erture » and l , he 



n J ,• iii image of courie 



central rays, will be as much larger than that formed by thefmaiier. 



rays of the circumference, as the focal diftance of the former 

 fhall exceed that of the latter. With a given aperture, the 

 focus will be that point in which the greateft number of rays 

 coincide, for, there, will the image appear moft diftind ; and, 

 confequently, as we enlarge the aperture of the lens, the 

 central rays muft be combined with others more convergent, 

 the new focus will approach the lens, and the image be, of 

 courfe, proportionably diminished. 



The total difagreement of this theorem with the refult ofThis is contrary 

 Mr. W.'s experiments, I can only account for by fuppofing^, ^. r ; 'Walker's 

 this gentleman, while he enlarged the aperture of his lens, he was probably 

 not to have paid attention to the increafing brightnefs of the deceived by the 

 .... .. „ . rr ,, ■ differences in 



image, which, by making a frronger lmpreihon on the retina, brightnefs. 



would appear to increafe in magnitude with every enlargement 

 of the aperture. If I might be allowed to offer my advice toPropofal that he 

 Mr. W. I fhould recommend him to repeat his experiments, fll0uld V **Y and 

 and fubftitule, for the flame of the candle, a fkreen having a riment. 

 circular opening covered with thin paper. This, with a light 

 behind it, may reprefent the horizontal moon. Some cha- 

 racters fhould be marked upon it, in order to determine, with 

 more facility, when the image is moft diftinct. The precau- 

 tion of reducing the different images, as nearly as poffible, 

 to the fame degree of brightnefs previous to meafurement, 

 muft not be neglecled. I am furprized indeed, that the 

 ftructure of the eye, which Mr. W. profeffes to have kept in 

 view in the courfe of thefe experiments, did not point out to 

 him the importance of this circumftance, which is fo admi- 

 rably anfwered, in the natural organ, by the contraction and 

 expaniion of the pupil. 



As the phenomenon of the horizontal moon has of late Phenomenon of 



much engaged the attention of feveral of your correfpondents, horia °ntal 



00 . moon occasioned 



I will avail myfelf of the prefent opportunity to add a few in part by the 



remarks on that fubject. It appears, at firft view, that the fu PP of,cion of 



.... ;, j a c . , . greater diftance, 



moft obvious method of accounting tor tne phenomenon, isby the aerial 



the following: We believe the moon more diftant from usP erf P e ^ iv€ » and 



when in the horizon than when in the meridian j but we ' ee ft arj e c r -" rCUm * 



it. 



