HISTORY OF THE MICROSCOPE. 3 



We shall first speak of the single microscope, that, as we have 

 already observed, having been invented and used long before the double 

 or compound microscope. When the lenses of the single microscope 

 are very convex, and consequently the magnifying power very great, 

 the field of view is small ; and it is so difiicult to adjust with accuracy 

 their focal distance, that it requires some practice to render the use of 

 them familiar. It was with an instrument of this kind that Leeuwen- 

 hoek and Swammerdam, Lyonet and Ellis, examined the invisible 

 forms of nature, laid open some of her hidden recesses, and by their 

 example stimulated others to the same pursuit. 



About the year 1665, small glass globules began to be occasionally 

 apphed to the single microscope, instead of convex lenses. By these 

 globules an immense magnifying power was obtained. Their invention 

 has been generally attributed to M. Hartsoeker ; though it appears that 

 we are really indebted to the celebrated Dr. Hooke for this discovery, 

 for he described the manner of making them In the preface to his 

 Microgra/phia lUustrata, published in the year 1656. 



Mr. Stephen Gray* having observed some irregular particles within 

 a glass globule, and finding that they appeared distinct and prodi- 

 giously magnified Avhen held close to his eye, concluded, that if he 

 placed a globule of water in which there were any particles more opaque 

 than the water near his eye, he should see those particles distinctly 

 and highly magnified. The result of this idea far exceeded his expec- 

 tation. His method was, to take on a pin a small portion of water 

 which he knew contained some minute animalcules ; this he laid on 

 the end of a small piece of brass wire, till there was formed somewhat 

 more than a hemisphere of water ; on applying it then to the eye, he 

 found the animalcules most enormously magnified ; for those which 

 were scarcely discernible with his glass globules, with this appeared as 

 large as ordinary-sized peas. Montucla observes, that when any ob- 

 jects are enclosed within this transparent globule, the hinder part of 

 the globule acts like a concave mirror, provided the objects be situated 

 between that surface and the focus ; and that by this means they are 

 magnified three times and a half more than they would be in the usual 

 way. 



An extempore microscope may be formed by taking up a small 

 drop of water on the point of a pin, and placing it over a fine hole 

 made in a piece of metal ; but as the refractive power of water is less 

 than that of glass, these globules do not magnify so much as glass ones 

 of the same size : this was also one of Mr. Gray's magnifiers. The 

 * Philosophical Transactions, 1696. 



