HO HORIZONTAL MOON. 



meafures on each page : and in this manner, I continued my 

 experiments until I had obtained five or fix meafures to each 

 aperture. This mode gave me an opportunity of feeing how- 

 much the flame of the candle altered in its length, during the 

 time that I was making the experiments, which alteration was 

 too fmall to be regarded. The meafures on each page being 

 added up, and divided by the number of them, gavemethofe 

 numbers which were inferted in my paper. 



But I did hot flop here, for fets of experiments were re- 

 peated in the fame manner, on feveral evenings, to afcertain 

 the fad more clearly. But to come more immediately to the 

 point; when the fun or moon was ufed inftead of a candle, 

 the fame refult was obtained, viz. the greater!: aperture gave 

 the greateft luminous pi&ure. 

 Principles of The truth of this property in optics, however, does not 



optics brought w | 10 n v re ft on mY experiments, the fame conclufion may be 



m fupport of . .. J ...... J 



the author's derived from other principles. 



theory. It is a we!! known property of the telefcope, that " as the 



light that enter aperture is contracted, the flender pencils or cylinders of rays 



\ the eye are that emerge from the eye-glafs into the eye, are alfo contracted 

 fmallei, the . .. r • „ «, 



fmaller the aper- in the ,ame proportion. 



of the ob- The magnifying power, of the object glafs of a telefcope 



riiie 



J ' is not increafed by increafing its aperture ; but by increafing 



its aperture, you increafe the magnitude of the pencil of light 



in its focus: for as the eye-glafs remains the fame, the increafe 



in magnitude of the emerging pencil of rays, mud depend 



upon an increafe in magnitude of the pencil in the focus of the 



—whence it is object glafs. Confequently, when the moon is viewed, if the 



inferred that the aperture of the object glafs be increafed, the picture of the 



will be equally mo°» within the telefcope, will be increafed in the fame ratio; 



changed in fize. but without increafing the magnifying power of the inftru- 



ment. 



For the magnifying power is =; the diameter of the aperture 

 of the object glafs divided by the diameter of the emerging 

 pencil, 



Detnonjl ration. 



Let A = the diameter of the aperture of the objeCt glafs, 

 B as the diameter of the emerging pencil, C = the focal diu 



* Elementary Parts of Smith's Optics, page 93. 



tance 



