34 PRACTICAL TREATISE ON 



the lens one-eighth of an inch. The least thickness possible 

 should be given to the glass composing the lens; the two 

 lenses of crown glass will be bi-convex, and the middle one 

 bi-concave, &c." This glass, however, appears never to have 

 been executed. 



In 1784, -^pinus made many fruitless trials to achromatize 

 the microscope, and, although he was successful to a certain 

 extent in destroying colour, he diminished rather than in- 

 creased the magnifying power of the instrument, and he made 

 it, says Adams, " rather more like a microscopic telescope than 

 a microscope." A blank now occurs in the pages of micro- 

 scopic history, from 1784 until 1800, and the microscopes 

 in use in those days were more remarkable for the improve- 

 ment in the mechanical construction of their stages and 

 adjustments, than for that of the optical part ; " and at 

 this period," says Chevalier, "it is to be remarked, with a 

 sentiment of regret, that England was more laborious than 

 Trance, and appeared to have the monopoly of the manufac- 

 ture of the best instruments."* 



From the year 1800 to 1810, we are told by Chevalier that 

 experiments were carried on by M. Charles, of the Institute, 

 to achromatize small lenses ; but the numerous imperfections 

 of these lenses were such, as to render their apphcation to the 

 microscope completely impossible, as they were not so ar- 

 ranged as to be cemented or superposed, and their centering 

 and curves, so full of imperfections, rendered them unfit for 

 microscopic purposes. 



In the year 1812, a very simple method was employed by 

 Sir David Brewster f to render both simple and compound 

 microscopes achromatic, which was as follows : — Starting -with 

 the principle that all objects, however delicate, are best seen 

 when immersed in fluid, he placed an object on a piece of 

 glass, and put above it a drop of some kind of oil, having a 

 greater dispersive power than the single or concave lens, 

 forming the object-glass of the microscope. The lens was 

 then made to touch the fluid, so that the surface of the 

 • Op. at. p. 85. 

 t Treatise on the Microscope, p. 73 et seq. 



