SULLIVANT AND 'SVORMLEY^ ON NOBERt's TEST-PLATE. 113 



contrary, their correctness seems to be expressly recognised 

 by Drf G. C. Wallich (' Ann. and Mag. Nat^i Hist.' for 

 February, 1860). 



Such being the conflicting testimony and opinion of dis- 

 tinguished microscopists on the capacity of the modern ob- 

 jective for separating lines, it is somewhat surprising — in 

 view of the high state of perfection now attained by the 

 microscope — that so few experiments have been made bearing 

 on this interesting point. 



As a contribution toward that object, we propose to offer 

 presently an analysis from actual measurements, as far as we 

 were able to carry them, of one of those '' marvels of art," 

 Nobert's test-plates. In such investigations the quality 

 of the instruments used being all important, we woidd state 

 that the optical apjDaratus at our command Avas ample, con- 

 sisting of a first-class Smith and Beck microscope-stand, 

 a ToUes' ^j'^yth objective of 160° angular aperture — an ob- 

 jective of rare excellence in all respects — besides ra^'is 

 and xV^lis of other eminent opticians, l)oth English and 

 American; also a solid eye-piece micrometer by ToUes, and 

 an. improved cobweb micrometer of Grunow's accurate work- 

 manship. Smith and Beck's stage scales furnished the 

 standards for fixing the micrometrical values of the eye- 

 pieces. By means of ToUes' amplifier, an achromatic con- 

 cavo-convex lens between the objective and the eye-piece, an 

 amplification (by the standard of 10 inches) as high as 6000 

 times was obtained. This high amplification, with sunlight 

 variously applied after passing through a small achromatic 

 lens of long focus, was effective in resolution, and essential to 

 the distinct counting nnder the micrometer of the lines of the 

 test-plate. The test-plate used consisted of thirty bands of 

 lines, each band varying but little from the —0^17^1^ of an 

 inch in width, and having its lines a uniform distance apart. 

 On one end of the plate is engraved by Nobert, in parts of 

 the Paris line, the distance apart of the lines composing the 

 first band, and thence on, tlie distance between the lines of 

 every fifth band, as in the 2d and 5th columns of the follow- 

 ing table : 



VOL. I. 



-NEW SEU. 



