6 HISTORY OP THE MICKOSCOPB. 



Leeuwenhoek fixed his objects, if they were solid, to these points 

 with glue ; if they were fluid, he fitted them on a little plate of 

 talc, or exceedingly thin-blown glass, which he afterwards glued to 

 the needle in the same manner as his other objects. The glasses 

 were ^11 exceedingly clear, and of different magnifying powers, pro- 

 portioned to the nature of the object and the parts designed to be 

 examined. 



He observed, in his letter to the Royal Society, that, from upwards 

 of forty years' experience, he had found the most considerable disco- 

 veries were to be made with glasses of moderate magnifying power, 

 which exhibited the object with the most perfect brightness and dis- 

 tinctness. Each instrument was devoted to one or two objects ; hence 

 he had always some hundreds by him. 



The three first compound microscopes that attract our notice are 

 those of Dr. Hopke, Eustachio Divini, and Philip Bonnani. Dr. Hooke 

 gives an account of his in the preface to his MierograpMa, published 

 in the year 1667 : it was about three inches in diameter, seven inches 

 long, and furnished with four draw-out tubes, by which it might be 

 lengthened as occasion required; it had three glasses — a small object- 

 glass, a middle glass, and a deep eye-glass. Dr. Hooke used all the 

 glasses when he wanted to take in a considerable part of an object 

 at once, as by the middle glass a number of radiating pencils were 

 conveyed to the eye which would otherwise have been lost ; but when 

 he wanted to examine with accuracy the small parts of any substance, 

 he took out the middle glass, and only made use of the eye and object 

 lenses ; for the fewer the refractions are, the clearer and brighter the 

 object appears. 



Dr. Hooke also gave us the first and most simple method of find- 

 ing how much any compound microscope magnifies an object. He 

 placed an accurate scale, divided into very minute parts of an inch, on 

 the stage of the microscope ; adjusted the microscope till the divisions 

 appeared distinct ; and then observed with the other eye how many 

 divisions of a rule similarly divided and laid on the stage were included 

 in one of the magnified divisions; "for if one division, as seen with one 

 eye through the microscope, extends to thirty divisions on the rule, 

 which is seen by the naked eye, it is evident that the diameter of the 

 object is increased or magnified thirty times." 



An account of Eustachio Divini's microscope was read at the Eoyal 

 Society in 1668. "It consisted of an object-lens, a middle glass, and 

 two eye-glasses, which were plano-convex lenses, and were placed so 

 that they touched each other in the centre of their convex surfaces ; 



