HISTORY OF THE MICROSCOPE. 9 



of making an object-glass composed of four achromatic compound 

 lenses, each consisting of two lenses. The focal length of each object- 

 glass was eighteen lines, its diameter six lines, and its thickness in 

 the centre six lines, the aperture only one line. They could be used 

 combined or separated. 



A microscope constructed on this principle, by M. Chevalier, was 

 presented by M. Selligues to the Academie des Sciences on the 5th of 

 April, 1824. In the same year, and without a knowledge of what 

 had been done on the Continent, the late Mr. TuUey, at the sugges- 

 tion of Dr. Goring, constructed an achromatic object-glass for a com- 

 pound microscope of nine-tenths of an inch focal length, composed of 

 three lenses, and transmitting a pencil of eighteen degrees ; this was 

 the first that had been made in England. 



It was at one time hoped, as precious stones are more refrac- 

 tive than glass, and as the increased refractive power is unaccompanied 

 by a corresponding increase in chromatic dispersion, that they would 

 furnish valuable materials for lenses, inasmuch as the refractions would 

 be accomplished by shallower curves, and consequently with diminished 

 spherical aberration. But these hopes were disappointed. Every 

 thing that ingenuity and perseverance could accomplish was tried by 

 Mr. Varley and Mr. Pritchard, under the patronage of Dr. Goring. It 

 appeared, however, that the great reflective power, the doubly-refracting 

 property, the colour, and the heterogeneous structure of the jewels 

 which were tried, much more than counterbalanced the benefits arising 

 from their greater refractive powers, and left no doubt of the supe- 

 riority of skiUully-made glass doublets and triplets. The idea is now, 

 in fact, abandoned ; and the same remark is applicable to the attempts 

 at constructing fluid lenses, and to the projects for giving to glass other 

 than spherical surfaces ; none of which have come into extensive use. 



Mr. Lister, who was engaged with Mr. Tulley in the perfecting 

 the achi-omatic object-glass, finding that none of the microscope-stands 

 hitherto made were sufficiently steady for the use of high powers, 

 directed his attention to the improvement of this part of the instru- 

 ment ; and in order to carry out his views, he employed Mr. James 

 Smith, now one of our first opticians, to execute a stand from his 

 own drawings, which he completed early in 1826. 



In March 1825 M. Chevalier presented to the Society for the En- 

 couragement of the Sciences an achromatic lens of four lines focus, 

 two lines in diameter, and one line in thickness in the centre. This 

 lens was greatly superior to the one before noticed, which had been 

 made by him for M. Selligues. 



