MAGNIFYING POWERS. 165 



been elevated from the condition previously described, to that 

 of being the most important instrument ever yet bestowed by 

 art upon the inv^tigator of nature."* 



In the doublet of Wollaston, and in the triplet of Mr. Hol- 

 land, before described, the lenses were all made of the same 

 kind of material ; but in order to render an object-glass for a 

 compound microscope achromatic, glass of two different den- 

 sities, and flint, crown, or plate, must be employed for each 

 lens, as in the telescope invented by DoUond. 



Glasses on this principle, of various degrees of merit, had 

 been made about the year 1824, by Selligue, Frauenhofer, 

 and Amici, on the continent, when William TuUey, df 

 London, without knowing what had been done by them, sud- 

 ceeded, in that year, in making the first English achromatic 

 object-glass for a compound microscope ; it was composed of 

 three lenses, and was capable of transmitting a pencil of 18° ; 

 he soon after constructed another combination, to be placed in 

 front of the first mentioned, which increased the angle of the 

 pencil to 38^; thus was the difficulty overcome. Mr. Tidley's 

 object-glass exhibited a flat field, and was perfectly corrected ; 

 to it also was applied an eye-piece, by which the magnifying 

 power produced was one hundred and twenty diameters, but 

 when the second combination was added, the power was in- 

 creased to three hundred. Mr. Tulley was encouraged and 

 greatly aided in his researches by Dr. Goring and Mr. Joseph 

 Jackson Lister, but still he never obtained, by his own com- 

 binations, an angular aperture beyond thirty-eight or forty 

 degrees : it was left for Mr. Lister, in 1829, to point out 

 how lenses composed of two kinds of glass, with their inner 

 surfaces in contact, could be combined so as to give, with ease 

 and certainty, a correction over the whole field, and at the 

 same time to transmit a much larger pencil. Mr. Lister's 

 paper, as before stated, at page 40, was published in the 

 Philosophical Transactions of the Royal Society, and so valuable 

 was the information afforded by it, that from the time of its 

 publication up to the present, it has been the foundation of 



* Penny Cyclopaedia, Art., Microscope. 



