134 STODDER, ON NOBERT's TEST-PLATE 



cal apparatus at their command Avas ample ; it included a 

 " Tolles' -5^0 objective of 160° angular ajD.erture — an objective 

 of rare excellence in all respects — besides ^-V and -J^ objec- 

 tives of other eminent opticians." They were able to obtain 

 an amplification of 6000 diameters. The plate contained 

 thirty bands, as previously mentioned. 



" Up to the 26th band (y^Trro) there was no serious diffi- 

 culty in resolving and ascertaining the position of the lines; 

 but on this and the subsequent ones, spectral lines, that is, 

 lines composed of two or more real lines, more or less pre- 

 vailed, showing that the resolving power of the objective 

 was approaching its limit. By a suitable arrangement, how- 

 ever, of the illumination, these spurious lines were separated 

 into the ultimate ones on the whole of the 26th, and very 

 nearly on the whole of the 27th band (-s-rn-nj-) ; biit on the 

 28th, and still more on the 29th, they so prevailed, that at 

 no one focal adjustment could more than a portion of the 

 width of these bands be resolved into the true lines. The 

 true lines of the 30th band we were unable to see, at least 

 with any degree of certainty. 



" These experiments induce ns to believe that the limit of 

 the resolvability of lines, in the present state of the objective, 

 is wellnigh established," and they draw the conclusion, 

 "that lines on the Nobert's test-plate, closer together than 

 about -g-TWo of ^^ iiich cannot be separated by the modern 

 objective." 



Although the paper of Messrs. SuUivant and Wormly 

 was republished in the ' Quarterly Journal of Microscopical 

 Science,' in London, and might be considered as being a 

 challenge to the opticians and microscopists of Europe to 

 show what they could do in resolving the test-plate, yet no 

 report can be found of any attempts to resolve the lines 

 until 1865, when Max Schultze (' Quart. Journ. Mic. Soc.,' 

 January, 1866) described the Nobert plate of nineteen bands, 

 and gave the results of his trials for resolving them. " The 

 highest set he has been able to define with central illumina- 

 tion is the 9th, which is resolved with Hartnack's immersion 

 No. 10, and Merz's immersion system ^^'t- With oblique 

 illumination he has not been able with any combination to 

 get beyond the 15th." It will be seen by reference to the 

 table that Schultze saw finer lines than Sullivant and 

 Wormly. This is the only rej)ort we can find in print 

 from Europe. 



In this country we find fto published results; but Mr. 

 R. C. Greenleaf, of Boston, and the writer were well 

 satisfied that they saw the lines 90,000 to the inch with a 



