6 



Microscopic a l 



Essays. 



the parts defigned to be examined. But none of thofe, which 

 were prefented to the Royal Society, magnified fo much, as the 

 glafs globules, which have been ufed in other microfcopes. He 

 had obferved, in a letter of his to the Royal Society, that from 

 upwards of forty years experience, he found that the moft con- 

 fiderable difcoveries were to be made with fuch glafles, as, magni- 

 fying but moderately, exhibited the object with the moft perfect 

 brightnefs and diftinclnefs. Each inftrument was devoted to one 

 or two objects : hence he had always fome hundreds by him. * 

 There is fome reafon for fuppoling, that Leeuwenhoek was ac- 

 quainted with a mode of viewing opake objects, fimilar to that 

 invented by Dr. Lieberkuhn. f 



About the year 1665, fmall glafs globules began to be occa- 

 sionally applied to the fmgle microfcope, inftead of convex lenfes. 

 By thefe globules, an immenfe magnifying power is obtained. 

 The invention of them has been generally attributed to M. 

 Hartfoeker ; it appears, however, to me, that we are indebted 

 to the celebrated Dr. Hooke for this difcovery ; for he defcribed 

 the manner of making them in the preface to his Micrographia 

 Illuftrata, which was publifhed in the year 1656. Now the firft 

 account we have of any microfcopical difcovery by M. Hart- 

 foeker, was that of the fpermatic animalcule, made by him 

 when he was eighteen years old; which brings us down to the 

 year 1674, long after Dr. Hooke's publication. 



As thefe glafs globules have been very ufeful in the hands 

 of experienced obfervers, I fhall lay before my readers the 



different 



* Philofophical Tranfa&ions, No. 380, No. 458. 

 ft Prieflley's Hiftory of Optics, p. 220. 



