208 Messrs. Sullivant and Wormley on Nobert's Test Plate. 



Such being- the conflictiog testimony and opinion of distin- 

 guished microscopists on the capacity of the modern objective 

 for separating hnes, it is somewhat surprising — in view of the 

 high state of perfection now attained by the microscope, and of 

 the number of its zealous devotees — that so few experiments 

 have been made, bearing on this interesting point. 



As a contribution towards that object, we propose to oflfer pre- 

 sently 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 instru- 

 ments used being all-important, we would state that the optical 

 apparatus at our command was ample, consisting of a first-class 

 Smith and Beck microscope-stand ; a Tolles's g'^-objective of 160° 

 angular aperture — an objective of rare excellence in all respects 

 — besides y^^^^ ^^^^ Tg^^^ o^ other eminent opticians, both En- 

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

 and an improved cobweb micrometer of Gruuow's accurate work- 

 manship. Smith and Beck's stage scales furnished the stand- 

 ards for fixing the micrometrical values of the eye-pieces. By 

 means of Tolles's amplifier (an achromatic concavo-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 

 under the micrometer of the lines of the test plate. The test 

 plate used consisted of 30 bands of lines, each band varying but 

 little from the 2 oui) ^^ ^^ ^^^^^ "^ 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, the distance be- 

 tween the lines of every fifth band, as in the 2ud and 5th columns 

 of the following Table : — 



We add the 3rd and 6th columns, giving- the distances in 

 parts of the English inch found by multiplying the decimals in 

 the 2nd and 5th columns by -088815. 



