SS Itlethod to delermhte the 



your readers, Ihave had manyopportunltiesof being convinced 

 that it is not fo generally known as it ought. Since I firft ven- 

 tured to give public lectures on optics, and to explain the va- 

 rious conftruftions of telcfcopes and microfcopcs, I have fre- 

 quently been applied to, to procure thofe inftruments for 

 people refiding at a diftance from the metropolis, and to 

 prove them before fending them off. I have alfo been fre- 

 quently requcfted by thofe who had purchafed fuch inflru- 

 ments to give an opinion of their goodnefs, and to determine 

 their magnifying powers ; from which I have found that the 

 manufacturers of thofe inftruments too generally are in the 

 habit of over- rating their powers *, to the great difappoint- 

 ment of the buyer, who finds, when too late, that they will 

 not perform what he hjid been taught to expeft from them. 

 From what I have ftated, it appeared to me, that a method 

 to determine the magnifying power of telcfcopes, without 

 much trouble or expenfe, and yet fuffieicntly exaft for moft 

 purpofcs, niight be thought worth a place iri your ufcful pub- 

 lication. There are various methods of doing the fame 

 thing, and each have their particular advantages and difad- 

 vantages : fome are cxpenfivc ; others require long calcula- 

 tions : fome are attended with much trouble; and others 

 require particular fituations, which cannot alv\ays be com- 

 manded. The method I moftly ufe can be univcrfally ap- 



" What is the method ufcd bv opticians to afcert;iin the magnifving 

 power of tclcfcnpcs ? and how dp tlicy piove how many times the objeft is 

 jTi.ignified, when feen through the faine ?" 



The prefent paper, he will fee, furniflies a fatibficlory anfwer. Edit. 



* Even that eminent artift Mr. Jamts Short, who rendered himfelf Co 

 juftly famous by bringing the reflefting telefcopc to a degree of perfeflion 

 ■vinknown before his time, too often ever-rated the power of his telefcopts, 

 though not in the fame fhamefui degree that fome makers now do. I 

 would not however be underftood as making this cenfurc genera): I know 

 cn^ artift whofe retiedting tele.topes exceed even thole of Short, and whole 

 telefcopes I have never once foL-nd cf lefs magnifving power tlian thtv 

 were fold for; and I hope there arc others why art above being gujliy of 

 (p raean an artince, 



pUedj 



