344 On the Catoptrical and Dioptrical 



By comparing the effects produced by chalk under the 

 first class, Experiment VI If, with this it appears evident 

 that calcareous earth produces the same facility to fusion 

 when mixed with siliceous matter as with argillaceous, and 

 the tendency of the iron to revive is nearly the same in both. 



LETTER IV. 



LVL On the Catoptrical and Dioptrical bistrnmcnts of 

 the Antientsx with Hints respecting their Revival, Re- 

 invention) or Improvement, in modern Times. 



DEAR SIR, 



1. Agreeably to what I promised in my last, I proceed 

 to give a brief account of two memoirs of father Abat, 

 which, though hot so interesting perhaps as that of which 

 I gave a translation in my last, tend, in a considerable de- 

 gree, to elucidate the present inquiry. I shall endeavour 

 to corroborate my author's statements with such proofs as 

 happen to be within my reach ; and shall conclude with a 

 concise recapitulation of what has been advanced. 



2. Abat's eighth Recreation is *j On a Mirror of Emerald, 

 in which the Emperor Nero viewed the Combats of the Gla- 

 diators " Having proved, from respectable antient autho- 

 rities, that Nero, the most detestable monster who had 

 then disgraced the name of emperor, was a myope, or short- 

 sighted, my author, whom the reader, no doubt, has by 

 this time discovered to be both learned and ingenious, ob- 

 serves, that in many myopes the organ of vision is so de- 

 licately sensible to the impression or light, as to require it 

 to be moderated before they can see distinctly ; that for this 

 purpose, some myopes view distant objects through small 

 holes, in thin plates of metal ; and that Nero, in order to 

 moderate the light, used a mirror, which my author argues, 

 I think with great appearance of truth, was nothing more 

 than an emerald reduced to a plane polished surface. 



3. I fear a detail of his arguments and authorities would 

 be thought tedious. But 1 cannot omit a curious fact 

 which he has taken from our countryman Ellis's Voyage 

 to Hudson's Bay; a work which, it would seem, is more 

 respected by the French than by ourselves; for I find my* 

 late excellent friend, Dr.Beattie, in his Dissertations, Moral 

 and Critical, regretting that so good a book should have been 

 suffered to go out of print. Speaking of the Esquimaux, (I 



translate 



