UHiat did our Forefathers see in a Microscope? 435 



(1830) gives 3 to 1 and 1£ for their distance apart, which was pro- 

 bably found to be the best proportions. It was stated that an 

 aperture as high as 0*42 N.A. was obtained, but I have measured 

 nothing over 0'3 N.A. 



Wollaston's Microscope had a plano-convex lens of about f in. 

 focus for a substage condenser. It was a very simple instrument, 

 quite small, the mechanical part being a brass tube 1 in. in 

 diameter, and about 6 in. long ; at the lower part there was an 

 aperture, and inside a plane mirror was fitted ; at the bottom there 

 was a screw to screw it either into a box lid for a foot or into a 

 screw-clamp for the edge of a table, just like his camera-lucida. 

 On the top of the tube was a small stage suitable for small slips, 

 say 1^ x \ ; a rack work coarse-adjustment was provided for 

 focusing the doublet. Subsequently this instrument was im- 

 proved, its most perfect form being attained in Vallentine's Micro- 

 scope, made in 1831 by Andrew Eoss.* This had sprung coarse- 

 and fine-adjustments ; the fine-adjustment head was graduated 

 for the first time, a rectangular mechanical stage, the limb holding 

 the doublet traversed the stage by means of a rack-and-pinion, and 

 could also rotate upon a conical pin. Subsequently it was found 

 that these doublets made very good object-glasses for a compound 

 Microscope, so a compound body was fitted to Vallentine's Micro- 

 scope, and thus Andrew Ross's earliest model of a compound 

 Microscope was formed. Vallentine's Microscope was afterwards 

 simplified : the fine-adjustment and the mechanical stage were re- 

 moved, but the sprung coarse-adjustment and the rackwork travers- 

 ing movement to the arm were retained ; it was with one of these 

 simplified instruments f made in 1839 that the experiments men- 

 tioned in this paper were carried out. Owing to the excellent 

 manner in which this coarse-adjustment is sprung (a method 

 adapted from a Microscope made by Powell for C. Varley), it works 

 to-day as well as ever it did — it is smoother, steadier, and softer 

 than any coarse-adjustment now turned out, either here or abroad. 



Two improvements were made in the optical part. The first 

 was in 1831 by J. Holland, who broke up the front lens into two, 

 thus making the first duplex front. The foci of one of Holland's 

 triplets are \, j^, and ^ in., all pianos ; the two last are in con- 

 tact, and between these and the back lens there is a diaphragm. It 

 was stated that the triplets performed better than the doublet, and 

 that an aperture as large as - 54 N.A. had been obtained. The 

 last improvement was by Chevalier, who, by making the lenses of 

 equal foci, with the lower lens of greater aperture than the upper, 

 increased the resultant aperture and much improved the doublet.^ 



* See this Journal, 1900, p. 425, fig. 104. 

 t Tom. cit., p. 428, fig. 109. 



X The date of Chevalier's improved doublet has not been determined : it may 

 have been about 1842. One of them is signed "Arthur Chevalier. Palais Royal." 



•2 G -J 



