SIZE OF THE HORIZONTAL MOON. f ^^ 



After Ihe ProfefTor had finifticd his celeftial exjilanation of 



uiis phenomenon, he juftiy acknowledges that, at diflTerent^ 



imes, the moon appears of very different magnitudes, even in 



the fame horizon, and occafionally of an extraordinary large 



fize, which he is not able to give a fatisfadlory explanation of. 



Smith's Optics, Vol. I. pa. 63, &c. Remarks, pa, 53. 



It is really aftonifliing, that this phenomenon fliould have re- Explanation of- 



mained fo long without an explanation founded on a better P""~po^ntion^that th 



ciple than that of mere opinion. That the dimenfiorjs of the images on the 



pupil of the eye alter by the ftimulus of light, has long been retina vary with 

 1 \ ^ , . , , , , thefizeotthe 



known; but I believe it ftill remains to be proved, that the pupil. 



pidure of the moon formed upon the retina is not pejmanent, 



but varies as the dimenfions of the pupil vary. The following ^^ ^^ ^ r^ 



experiments will, I hope, remove every doubt refpecling this " » 



law of vifion. 



To imitate the eye upon a large fcale, I took an achromatic Experiment. 



lens of 1.6 inches in diameter, and 17 inches focal diftance, to placed bef^e^a 



reprefent the cryftalline humour. This I fixed upon a fland in achromatic lens, 



a perpendicular direction, with a moveable white fcreen be- ^"^ '^^''^^ ^P"" 

 , • 1 • r 1 • • 1 • r I- , 1 ., tures were pro- 



hmd It for the retina, to receive the image of a lighted candle, vided. 



which ftood before it, at the diftance of 40 inches. To imitate 



the pupil, I took three cards, and numbered them 1, 2, and 3, 



In No. 1, I made a circular hole half an inch in diameter, in 



No. 2 a hole of -^-^ of an inch, and in No, 3, a hole of -^ 



of an inch. 



After I had moved the fcreen until the image of the candle Focal image by 

 appeared diftindly upon it, I found that the length of this f Ij^'^^hi,"" 

 luminous picture meafured 1.82 inches. Jong. 



When the card No. 1, was applied clofe to the lens, with Half an inch 



the centre of the hole againft the centre of the lens, the in- aperture gave an 



*=• , image of 1.46 



verted image of the flame of the candle, upon the fcreen, inch. 



meafured 1.46 inches. 



When the card No 2, was applied to the lens, in the fame Three tenths 

 manner as No. \, the image of tlie flame meafured 1.3 gave 1.3 inch, 

 inches. 



And with No 3, the picture of the flame meafured only 1.2 Two tenths gave 

 inches. 1.2, inch. 



Thefe experiments lliow us, in thecleareft manner, thecaufe Application of 



of that variation which obtains in the apparent raajinitudes of ^^^ "P"''"^"*^. 



^ to the moon ana 



ODjetls, other obiefts. 



That 



