16 Microscopical Essays. 



The three firft compound microfcopes that attract our notice, 

 are thofe of Dr. Hooke, Euftachio Divini, and Philip Bonnani. 

 Dr. Hooke gives an account of his in the preface to his Micro- 

 graphia, which was publifhed in the year 1656; it was about 

 three inches in diameter, feven long, and furnifhed with four 

 draw-out tubes, by which it might be lengthened as occafion re- 

 quired : it had three glaffes, a fmall object glafs, a middle glafs, 

 and a deep eye glafs : Dr. Hooke ufed all the glaffes when he 

 wanted to take in a considerable part of an object, at. once, as by 

 the middle glafs a number of radiating pencils were conveyed to 

 the eye, which would otherwife have been loft : but when he 

 wanted to examine with accuracy the fmall parts of any fubftance, 

 he took out the middle glafs, and only made ufe of the eye and 

 objecl: lenfes ; for the fewer the refractions are, the clearer and 

 more bright, the objecl appears. 



An account of Euftachio Divini's microfcope was read at the 

 Royal Society, in i658. * It confided of an objecl; lens, a mid- 

 dle glafs, and two eye glaffes, which were piano convex lenfes, 

 and were placed fo that they touched each other in the center of 

 their convex furfaces ; by which means the glafs takes in more of 

 an objea, the field is larger, the extremities of it lefs curved, 

 and the magnifying power greater. The tube, in which the 

 glafles were inclofed, was as large as a man's leg, and the eye 

 glaffes as broad as the palm of the hand. It had four feveral 

 lengths ; when {hut up, it was fixteen inches long, and magnified 

 the diameter of an objed forty-one times ; at the fecond length, 

 ninety times ; at the third length, one hundred and eleven times ; 



at 



* Philof. Tranf. No. 42. 



