452 



♦ KNOV^LEDGE • 



[IIat 29, 1885. 



of water, bold it above eye level, look through the side to 

 the water surface. The air that rests upon it appears like 

 burnished silver, and its opacity is proved by holding the 

 finger or any other object just above the water surface, 

 then dipping it in the water. This simple experiment is 

 very instructive. 



In my next I will endeavour to supply a proximate idea 

 of the quantity of air that may possibly be held captive by 

 clothing materials. 



ROBERT'S RULIXG MACHINE.* 



Br John- IMayall, Jux., F.E.il.S., F.Z.S. 



{Continued from p. 433.) 



ABOUT the year 1823 a great improvement in the 

 construction of compound refracting microscopes was 

 initiated by Selligues,+ who applied achromatism to the 

 objectives, by which, in the course of a few years, the 

 instrument was so much advanced that there was consider- 

 able difficulty in discovering test-objects of a nature 

 suitable to serve as standards for estimating the quality of 

 the newer objectives, and especially for testing the value 

 of each increase of aperture as successively attained. 



Herr Xobert appears early iu life to have been interested 

 in the progress of the construction of the achromatic re- 

 fracting microscope. The difficulty attending the testing 

 of the increased apertures, doubtless, led him to devote his 

 attention to the production of fine rulings, whilst the 

 publication of Frauenhofer's experiments with diffrac- 

 tion gratings would likewise stimulate him to improve 

 upon the results obtained with Frauenhofer's rulings, both 

 in the direction of fineness and accurac}-. 



For some time previous to lSi-5. Herr Xobert, then 

 resident in Greifswald, appears to have been engaged in 

 designing and constructing the ruling machine which I here 

 exhibit, for in that year he succeeded in ruling a test-plate 

 with ten bands of lines, which, commencing with -j J--th 

 of a Paris line, and continuing in geometrical progression, 

 ended with lines of ^^^L-jth of a Paris line (from yy'-^-j^ to 

 ^ 5 ;, ^ 5 of an English inch). You will observe" that 

 Robert's coarsest lines ranged somewhat finer than the 

 finest of the set of micrometer- plates I have referred to as 

 representing the fine rulings before the days of the achro- 

 matic refracting microscope. Each band ruled by Xobert 

 consisted of a number of parallel and equidistant lines at 

 the rate specified, and a blank space, about equal in breadth 

 to the bands, separated the successive bands. These bands 

 of lines were intended as test-objects, to determine the 

 resolving power of microscope-objectives. Herr Xobert 

 mentions (in a ilSS. published after his death, but 

 evidently written about 1861) that "the German micro- 

 scopes of that dates resolved the 8th band of that plate, 

 and Amici's the 9th, whilst in the followiog years, when 

 the plate had become known in England, the decided 

 superiority of English instruments resolved all ten bands." 



The immediate effect of Robert's rulings was to reverse 

 the order of things that had hitherto obtained in micro- 

 scopy. Before the issue of his test- plate, opticians and 

 microscopists hardly knew where to seek for an object 

 capable of severely testing the resolving power of their newer 

 objectives with increased apertures. But now, at one bound, 

 he went beyond the resolving power of the finest microscopes 



of Arts. A paper read before 



* From the Jourj 

 the Society on Hay 6. 



+ Quekett's " Treatise on the ilicroscope," third edition (1853), 

 p. 40. 



of the day. And, strange as it may seem, so far reach- 

 ing was Nobert's mechanical genius, that the machine of 

 18-1.5 not only enabled him to produce rulings finer 

 and finer, always beyond the resolving power of the most 

 perfect objectives as successively brought out by the con- 

 temporary opticians: but when, in li>G9, after most ardent 

 devotion to the task, the eminent microscopist, the late 

 Dr. J. J. Woodward, of the Army Medical Museum, 

 Washington, succeeded in photographing the highest band 

 of the 19 -band plate ( mooo th of a Paris line= ^^^'^^, of 

 an English inch, with Powell and Lealand's water-im- 

 mersion -j'^th objective, Xobert immediately outstripped 

 all his previous efforty by ruling a plate with lines twice 

 as fine — that is to say, ranging from -^--'— th to . ,^} , , th of 

 a Paris line (from xt'^t '•° ^^-^^^^ °^ ^^ English inch). 

 This last plate was rather a tour deforce, and will tax the 

 resolving power of the microscopes of the future. 



By the courtesy of the Surgeon-General of the United 

 States Army I have received the series of Dr. Woodward's 

 photo-micrographs of Xobert's 19-band plate, and they are 

 here for your inspection. 



But I must go back again briefly to earlier dates. About 

 18.30, Mr. Warren De la Rue* made an elaborate series of 

 trials with Xobert's lines, and induced him to add bands of 

 finer lines to his test-plates, so that gradully he arrived at a 

 plate containing 30 bands, beginning at ^rr-.-p^th and reaching 

 ^— pth of a Paris line (^=-y—}rg7X^ of an English inch). 

 In 1860, Messrs. Sullivant i Wormley '^ succeeded in 

 coimting the lines in the 30th band, whereupon Xobert, 

 wishing to add still finer lines, and yet to diminish the 

 labour of ruling so many bands, recast the frac- 

 tional divisions of the successive bands, still keeping 

 to the Paris line as the unit, resulting in the 19-band 

 plate to which I have already referred. When Dr. Wood- 

 ward succeeded in photographing and counting all the lines 

 on the 19-band plate, thus furnishing a perfect demon- 

 stration that the development of the resolving power of 

 the microscope had unquestionably reached the level of the 

 19-band plate, Xobert again re-cast the fractional divisions 

 of the successive bands, resulting in the latest 20-band plate 

 ranging from -j-J^^^th to ., , , I ^ -^ of a Paris line. Regarding 

 this plate, I may remark that Dr. Woodward did not 

 succeed in photographing any lines beyond the 19th band, 

 which was precisely equivalent to the 19th of the 10-band 

 plate. I take this opportunity, however, of stating that 

 Mr. E. M. Xelson, so well-known for his manipulative 

 skill with the microscope, has succeeded in resolving the 

 eleventh band ( = ^J-~ of a Paris line ^ i .^ 3'^ ; 3 of an 

 English inch), using the oxy-hydrogen lamp for the illumi- 

 nation in my presence. 



You will understand, then, that Herr Xobert devised 

 these fine rulings to serve as test-objects for the resolving 

 power of microscope-objectives. On the supposition that 

 an objective had a given extent of aperture free from aber- 

 ration, then, according to a certain optical formula, 

 suggested by Frauenhofer, such an c.bjective should resolve 

 consecutive lines of a certain degi-ee of closeness. The 

 bands of lines were graduated in fineness by Xobert, with 

 a view to furnishing an approximately exact measure of 

 the limit of the resolving power of any objective tested 

 upon them. In recent years Frauenhofer's formula has 

 been revised by Helmholtz and Abbe (independently), so 

 that the theoretical limit of resolution has been extended 

 two-fold for every increment of aperture, on the supposi- 

 tion that light of the greatest possible obliquity is employed 

 for the illumination. 



* Qaeckett's "Treatise on the Microscope," third edition (1855), 

 pp. 511-4. 

 I -f- " American Journal of Sciences and Arts," Jan., 1861. 



