IMAGES FORMED BY CONVEK LENSES. 277 



•,nat it L> fcarcely poflible to form a new one. This corre- 

 fpondent has picked up the theory of Des Cartes, which was 

 confuted more than 100 years ago by Mr. Molyneux. Sec 

 Philof. Tranf. abr. Vol. I. p. 221; or Dr. Hutton's Mathe- 

 matical Dictionary, Vol. II. p. 74-. I fhall pafs over this 

 writer's optical errors by juft obferving, that he has been 

 doing nothing Ieis than denying an eftablifhed property in 

 optics. 



But before I proceed, it may be convenient to thofe who Farther remarks 

 are not in podellion of your two Numbers which contain ray° n .'^ 



* ' J formed by a Jens, 



former papers on this fubjefr, to know what that property 



in optics is, which has met with fo much oppofition. 



I have proved experimentally, that the image of a candle afferted to vary 



in the focus of a double convex lens, decreafes m magnitude*" c a ? er ~ 



* ft ture* 



as the aperture of the lens is contracted*: And I have alio 



advanced, that the image of the fun or moon, in the focus 

 of the objeQ-glafs of a telefcope, decreafes with its aper- 

 ture f . 



Now it is well known to thofe who underftand optics, This pofnion 

 that this property, the truth of which has been denied fo po- a ' crl eJ toN2W * 

 iitively, was difcovered by Sir Ifaac Newton : and a demon- 

 ftration of it may be feen in the Optics of Newton, Emerfon, 

 Smith, and Martin, and alfo in the Encyclopaedia Britan- 

 nica, &c. 



" The breadth of the lead circular fpace into which object- becaufe he notes 

 glafles of telefcopes can collect all forts of parallel rays, | s the (pace of dif- 



i i r rii/-i /•!!/- fu lion in pencils 



about the 27 ■§ part ot halt the aperture ot the glafs, or 5.3th r rays. 



part of the whole aperture." (Newton's Optics, 2d Edit. 



p. 73). Hence it is evident, that if you contract the breadth Whence it is 



of the objecWlafs from 10 to 1, the breadth of the circle of d f iuced ' th f 

 .... ' r the imagp alfo 



light, in its tocus, will be contracted in the (ame proportion. w ii} vary. 



In my next paper, this interefting property of light will 



be further explained, 



I am, Dear Sir, 



Your humble fervant, 



EZ. W/ LKF.R, 



Lynn, March 16, 1805. 



* Philofophical Journal, Vol. IX. p. 164. 

 f Fhilufopmcal Journal, Vol. X. p. 110. 



