228 E. M. NELSON ON HISTORIC MICROSCOPY. 



in one combination or object glass. The focus of each of the com- 

 pound parts varied from 1-| to l£ inch, but the convex sides of the 

 lenses were turned towards the object, so that the spherical aberra- 

 tion of the combination was at its maximum. 



In 1825, M. Chevalier made lenses of less focal length, and 

 turned the plane side to the object, which was a great advance on 

 the previous method. His highest power was a combination of 

 two T %. 



In the same year, 1825, Mr. Tulley, with the assistance of Mr. 

 Joseph Jackson Lister, made a triplet of something less than one 

 inch in focus and 18° in aperture. Afterwards he made another 

 of still shorter focus, which, when combined with the first, in- 

 creased the aperture to 38°. This combination is said to have 

 stood a power of 300 diams. very well. Andrew Ross, speaking 

 of these triplets, said, " that they never have been exceeded by any 

 similar combination for accurate correction throughout the field." 



Prof. Amici, in 1827, brought a horizontal microscope to this 

 country with a triple achromatic objective. 



In 1830, J. J. Lister read a paper before the Royal Society on 

 " Some Properties in Achromatic Object-glasses Applicable to the 

 Improvement of the Microscope." From the lines laid down in 

 this paper, Ross, Powell, and James Smith made objectives which 

 surpassed any made elsewhere. 



In conclusion, let me point out how the extraordinary instru- 

 ments I have already described got transformed into those of the 

 present day. 



Andrew Pritchard's compound refracting microscope consisted 

 of a telescope tripod clip stand with compass joint for inclination, 

 a Tyrrell's stage, a condensing lens on substage, and a mirror. 

 The focussing adjustments of this instrument were peculiar. The 

 arm carrying the compound body was fixed to a tube which fitted 

 in the tube forming the tail-piece, and came out of the end of the tail- 

 piece. The coarse adjustment was effected by pushing this tube in 

 and out. At the end of this tube was a milled head, which, when 

 turned round, worked a direct acting screw fine adjustment, in a 

 manner similar to the ordinary Hartnack, only it was at the end of 

 the tail-piece instead of in the usual place. 



In 1826, Mr. James Smith made for Mr. Joseph Lister a micro- 

 scope on the following plan : — Folding tripod stand with single 

 pillar, compass joint. The compound body slides in a tube 



