26 PRACTICAL TREATISE ON 



could exhibit these objects well, was considered a proficient in 

 the art. These instruments, as described by Adams, without 

 any material alteration in the optical part, continued in 

 use up to the time of the invention of the achromatic form, 

 in 1824, but a new and most important jera in microscopic 

 science commenced in this country with the improvement 

 in the reflecting microscope, constructed by Amici in 1815, 

 and with the manufacture of lenses of the precious stones 

 by Sir David Brewster, Dr. Goring, and Mr. Pritchard. 

 At this period it will be necessary to divide our history into 

 two parts. The first to include the improvements made in 

 the single, and the second those in the compoimd micro- 

 scope. In consequence of the great loss of light, and the 

 presence of the prismatic halo enveloping every object seen 

 through the uncorrected compound microscope, the single 

 microscope was generally used by all scientific investigators ; 

 but when high powers were wanted, the glass of which 

 they were made being of such low refractive power, it 

 became necessary to use lenses of very short foci, these 

 were of very small diameters, and allowed only a slight 

 amount of light to enter the eye; to remedy these incon- 

 veniences. Sir David Brewster first suggested the value of 

 usiag other materials of a more highly refracting nature, for 

 the construction of lenses; and he remarked,* ''that no 

 essential improvement could be expected in the single 

 microscope, unless from the discovery of some transparent 

 substance, which, like the diamond, combines a high re- 

 fractive with a low dispersive power. Having experienced 

 the greatest difficulty in getting a small diamond cut into 

 a prism in London, he did not conceive it practicable to 

 grind and polish a diamond lens ; and, therefore, he did not 

 put his opinion to the test of experiment, but he got two 

 lenses, one made of ruby, the other of garnet, which he found 

 to be greatly superior to any lenses that had previously been 

 used." Dr. Goring, in the summer of 1824, having directed 

 the attention of Mr. Pritchard to certain passages in Sir 

 David Brewster's admirable Treatise on New Philosophical 

 " Treatise on the Microscope, p. 13. 



