J40 APPARENT ENLARGEMENT OF THE MOON, 



the horizontal moon above what it has in the zenith; and 



therefore prefent 3*011 with the idea I have entertained upon 



the fubjecl. 

 Eftimate of mag- J ain f pi n ; on t] ia t the magnitude of any object can only 

 to the foe of the De eftimated (as to fenfe) according to the fpace or magnitude 

 image on the of its image upon the retina. The eye muft (if this is ad- 

 xetma. mitted), therefore, be the fole caufe of the illufion, for the 



moon's apparent magnitude increafes as fhe rifes towards the 



zenith. 

 The eye is form- The eye by the power of the mufcles can, at the will of the 

 ednot only to bf erver no t only be direded towards the object, in order to 



alter its aperture, ' J ■> * 



but alio its focal receive its image upon the ufual part of the retina, but acquire 



«hftance. a p r0 per convexity for diftinct vifion, or focus of the refracted 



Elongation or ■ * " - 



the eye will en- rays upon the retina ; and alfo to contract or extend the area 

 large the image, f the pupil according to the quantity of light. This admi- 



though the an- , , . . ,r j, ■ rr c 



gular magnitude raD ' e property may be oblerved by every one in palling out ot 

 remains twial- a light room in the night, and alio in the eyes of the brute 

 creation ; but I conceive, that while the area of the pupil is 

 enlarged in order to receive a greater quantity of light, the 

 eye is protruded, or the focal diftance between the pupil and 

 retina is increafed, which increafes the fenhble magnitude of 

 the object, as the image upon the retina conveys it to our 

 fenfes, when viewed by the naked eye. This I diitinguifli 

 from apparent magnitude, which we ufe when actually mea- 

 sured by an inftrument. 

 Variation of the The variation in the magnitude of the image upon the 

 pupil, or of the retina, certainly cannot anfe from the variations in the dimen- 

 lens, cannot ^ ons ov " '' ,e ptt pfl , which is contrary to the law of dioptrics; 

 alter the image. f or> on oblerving the fun, I do not admit light through 

 one-twentieth part of the area of the object glafs, which is 

 3.S -inches in diameter, and yet there is not any difference in 

 the apparent diameter ; or, whether the rays are admitted at 

 the center or any other part of the object-glafs, the obferva- 

 The elongation tions are equally good. But I confider it as a natural confe- 

 ©f theeyeis fup- q U ence, that when the pupil of the eye is enlarged, the focal 

 ^\ Z - whenever Pittance ' s increafed, and which conftanlly arifes from adimi- 

 ths pupil is en- nution of light, even without our attention ; but thereby our 

 ,ai 'S eJ i fight is affifted in confequence with more light and power, 



which is fome compenfation. 

 and as this takes T' ie I'g' 1 *- ' rom a celeftial objecl near the horizon, is dimi- 

 place when the niflied by palling a long diftance through a grofs atmofphere ; 

 jnoonisleaftlu- «. hcncc 



