MifcelJanea Curioja. 357 



For? the Rule is ^£ =3 



pdr-hdf-t-frf 



But for p will be ~ * * ? 7 = />, which 



latter determines the fyitio of Refraction, w being 

 to w,as 1 -f- p to pi 



I (hall not expatiate on thefe Particulars, but 

 leave them for the Exercife of thofe that are 

 defirous to be informed in Optical Matters, 

 which I am bold to fay are comprehended in thefe 

 three Rules, as fully as the mofl: Inquifitive 

 Kan defire them, and in all poflible Cafes ; 

 regard being had to the Signs -± and — , as in 

 the former Cafes of finding the Focus. I fhall 

 only fhew two confiderable Ufes of them ; the 

 one to find the diftance whereat an Object being 

 plac'd, fhall by a given Lens be represented in a 

 Species as large as the Object: it felf, which may 

 be of fingular life in drawing Faces and other 

 things in their true Magnitude, by tranfmitting 

 the Species by a Glafs into a dark Room, which 

 will not only give the true Figure and Shades, but 

 even the Colours themfeives, almoft as vivid as 

 the Life. In this Cafe d is equal to/, and fub- 

 ftituting d for / in the Equation, we fhall 

 have p dr ? = d dr -+ d d ? — dp fr, and di- 

 viding all by dpr§=.dr~i-d ?— p r f, that is^ 



d \ but if the Convexities be of 



r *+• § 



the fame Sphere fo as r = f, then will the di- 

 ftance be = f r ' y that is, if the Lens be Glafs 

 = ^ r, fo that if an Object be placed at the 

 Diameter of the Sphere diftant , in this Cafe 

 the Focus will be as Jar within as the Object is 

 A a >% with 



