THE MICROSCOPE. 



11 



plan of making lenses by letting drops of water fall on pieces 

 of plane glass which form themselves into planoconvex lenses, 

 and he found that they magnified greatly; but as the fluidity 

 of the water obliged him to keep the glass horizontal, he was 

 led to try isinglass dissolved in hot water, whereby the drops, 

 when cold, although less transparent than the pure water, 

 nevertheless allowed of these lenses being used in any position, 

 a plan which many years after was followed up and greatly 

 improved by Sir David Brewster, who employed minute drops 

 of varnish or other viscid fluids placed on the thin pieces of flat ' 

 glass. When the lens so formed was required to be very 

 convex, the glass was held so that the drop was downward ; 

 but when less convex, then the drop was allowed to dry with 

 the plate of glass downwards. 



In the year 1702, we find in the Philosophical Transactions 

 a description of the pocket microscope of Mr. J. Wilson, 

 who, following the opinion of Hooke, that single magnifying 

 glasses, when they can be used, are preferable to microscopes 



composed of two or more 



magnifying glasses, was led 

 to the construction of this in- 

 strument, which, from its fre- 

 quent mention by Baker and 

 other authors, appears to 

 have had a far-famed cele- 

 brity, and, indeed, many 

 specimens of it are still to 

 be met with ; one of the ear- 

 liest forms of this instrument 

 is represented by fig. 8. The 

 body, A A A A, which was 

 made either of ivory, brass, 

 or silver, was of a cylindrical 

 figure, and about two inches 

 in length, and one inch in 

 diameter ; to the lower end, B, 

 the magnifiers are adapted, 

 whilst into the upper screws a piece of tube, D, having at 



