Microscopical Essays, 



55 



The foregoing rule has been alfo applied to eftimate the mag- 

 nifying power of the compound microfcope. To this application, 

 Mr. Magny, in the * Journal d'Economie pour les mois d'Aout 

 1753," has made feveral objections : one or two of thefe I mall 

 juft mention : the firft is the difficulty of afcertaining with accu- 

 racy the precife focus of a fmall lens ; the fecond is the want of a 

 fixed or known meafure, with which to compare the focus when 

 afcertained. Thefe confiderations, though apparently trifling, 

 will be found of importance in the calculations which are relative 

 to deep magnifiers. To this it may be further added, that the 

 fame ftandard or fixed meafure cannot be aflbmed for a fhort- 

 fighted, that is, ufed for a well-conftituted eye. To obviate 

 thefe difficulties, and fome errors in the methods which were 

 recommended by Meff. Baker and Needham, Mr. Magny offers 

 the following 



Proposition. 



All convex lenfes, of whatfoever focus, double the apparent 

 diameter of an object, provided that the object is at the focus of 

 the glafs on one fide, and the eye is at the fame diftance, or on 

 the focus of the glafs, at the oppofite fide. 



Experiment. 



Take a double convex lens, of fix or eight inches focus, and 

 fix it as at A, Fig. 1, (Plate II. A,) into the piece A, which is fixed 

 perpendicular to the rule F G, and may be Hid along it by means 

 of it's focket : the rule is divided into inches and parts. Pa fie a 

 piece of white paper, two or three tenths of an inch broad, and 

 5 three 



