8 HISTORY OP THE MICROSCOPE. 



Bonnani. Dr. Hooke gives us an account of his in the 

 preface to his Micrographia, 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," he writes, lt the 

 fewer the refractions are, the clearer and brighter the 

 object appears." 



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

 of finding 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 Royal 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. The tube in 

 which the glasses were enclosed was as large as a man's 

 leg, and the eye-glasses as broad as the palm of the hand. 

 It had four several lengths : when shut up was 1 6 inches 

 long, and magnified the diameter of an object 41 times, 

 at the second length 90, at the third length 111, and at 

 the fourth length 143 times. It does not appear that 

 Divini varied the object-glasses. 



Philip Bonnani published an account of his two micro- 

 scopes in 1698. Both were compound. The first was 



