6 Mr. T. D. Aldous o» the 



plates, and by sliding this up and down the object was brought 

 to focus. 



There were other modifications of the various instruments I 

 have mentioned, but I think I have taken the chief types ; and of 

 these I have prepared some diagrams, to which I would now 

 direct your attention. 



Of the double or compound microscope, consisting of two, 

 three, or more lenses, the earliest recorded examples are those 

 of Dr. Hooke, 1665 ; Eustachio Divini, 1668 ; and Philip Bonani, 

 1698. 



Dr. Hooke's compound microscope was composed of three 

 lenses, the object-glass, a middle or field lens, and an eye- 

 glass ; it was about three inches in diameter and seven long, 

 and was furnished with four draw tubes. In his work, ' Micro- 

 graphia,' Dr. Hooke gives a method of examining opaque objects 

 by placing a globe of glass filled with brine immediately in front 

 of his lamp, the pencil of light from this globe bemg received 

 by a small plano-convex lens with its convex surface turned 

 towards the globe, thus concentrating the rays on to the object, 

 which, I need hardly mention, is practically the same as that 

 used to-day. Dr. Hooke states that when he wished to examine 

 an object with greater accuracy, he removed the middle glass, 

 by which means he obtained more light and better definition. 



The microscope of Eustachio Divini must have been a some- 

 what remarkable instrument ; it, like that of Dr. Hooke, con- 

 sisted of an object and field glass, but instead of a double convex 

 eye-glass, he substituted two plano-convex lenses, which touched 

 each other at the centre of their convex surfaces. This was an 

 improvement, as by this arrangement a flat field was obtained 

 with a considerable increase of magnifying power. By means 

 of four draw tubes Divini obtained amplifications of from 40 to 

 143 diameters. When the draw tubes were all closed the body 

 was 16 in. long, and magnified 41 times, at the second length 90, 

 at the third 111, and at its fourth and greatest expansion 143; 

 the body-tube was a large as a man's thigh, and the eye-glass 

 was equal in size to the palm of the hand. Altogether it must 

 have been an imposing instrument, 1668. 



When writing the above I much regretted being miable to 

 show you a drawing of this instrument. What, then, was my 

 delight the other day to find that almost as I was writing, 

 Sig. P. A. Saccardo found preserved in the Museo di Fisica, 

 Padua, an ancient microscope bearing the inscription, "Eustachio 

 Divini in Roma, 1672." A drawing of this instrument appeared 

 in the December number of the ' Royal Microscopical Journal,' 

 and to that most excellent publication I am indebted for the copy 

 of the enlarged sketch now before you. 



The measurements of this instrument are as follows : — When 



