July 31, 1913] 



NATURE 



559 



THE UNPUBLISHED PAPERS OF 

 J. J. LISTER. 



A LARGE parcel of miscellaneous papers by 

 J. J. Lister, and some pieces of apparatus, 

 were left to the Royal Microscopical Society by the 

 late Lord Lister. The papers were eventually 

 submitted to me for examination, and this resulted 

 in the discovery of the important paper on the 

 limit of defining power in vision with the un- 

 assisted eye, the telescope, and the microscope 

 now published in the society's journal. 1 



The underlying remarkable experiments were 

 made, and a first — still existing — MS. prepared in 

 1 83 1-2, shortly after Lister had published his 

 famous paper on the improvement of the achro- 

 matic compound microscope (Phil. Trans., 1830, 

 pp. 187-200J. The manuscript was entirely re- 

 written, practically as now published, in 1842-3, 

 and again revised in 1853; but although the 

 author lived until 1863, he never published this 

 excellent piece of work, which is interesting and 

 instructive even now, fully eighty years after the 

 doing of it and just fifty years after the author's 

 death. 



The original paper is now so easily accessible, 

 and is so well worth reading, that instead of giving 

 a detailed account of the contents I propose dis- 

 cussing more particularly those of Lister's results 

 which are either of real interest even at the present 

 time, or open to criticism as to their validity. 



The most striking feature of the work — and un- 

 doubtedly so intended by the author — is its abso- 

 lute homogeneity ; for precisely the same objects 

 were used in experimentally determining the limit 

 of resolving power of the naked eye with full 

 aperture and through circular apertures down to 

 0*00059 in. diameter, of telescopes of various aper- 

 tures up to 4 in., and of a large number of good 

 microscope objectives covering a wide range of 

 angular apertures. 



The objects, which are still in existence, being 

 included among those now in the keeping of the 

 Royal Microscopical Society, were very accurately 

 made coarse gratings, produced by sticking paper 

 on glass plates, cutting parallel lines through it, 

 and removing strips at equal intervals. There 

 was also a similarly produced chess-board pattern 

 on glass, the separation from line to line or square 

 to square being of the order of 1/10 in. This 

 coarseness of the objects leads to the principal 

 point worth discussing ; for, as only 400 ft. 

 distance was available, the objects could not be 

 observed directly, even in the telescopic experi- 

 ments, a diminished image of the actual object 

 in a convex mirror having to be used instead. 



In the microscopic observations a very greatly 

 diminished image was employed which an auxiliary 

 objective of higher power and wider angular aper- 

 ture than that to be tested projected in the 

 common focal plane of both. To most people 

 this will appear as a perfectly legitimate proceed- 

 ing involving only the most elementary optical 

 assumptions and therefore not open to objection. 

 But a crude method of carrying out this process with 



1 rourn. R. Micr. Soc. ic,i 3 , pp.. 



NO. 2283, VOL. qi] 



the microscope by observation of the small images 

 formed by air-bubbles or fat-globules in a watery 

 liquid has been severely criticised by Abbe for 

 two reasons : first, because such images, when 

 received by microscope objectives of wide aperture, 

 must be affected to such an extent by aberrations 

 as to render any calculation as to their size and 

 nature utterly futile. Little can be said against 

 this, but it does not apply to Lister, as he takes 

 great care to state that the projecting lens was 

 a perfectly corrected and carefully adjusted micro- 

 scope objective. But Abbe raised a second objec- 

 tion of more serious aspect : — 



" Even supposing that a perfectly corrected pro- 

 jection-system be used, the observation is not 

 really a microscopical one at all ; it is a quasi- 

 telescopic observation of the actual object by an 

 instrument giving erect images, consisting of the 

 projecting system and the real microscope, which 

 latter acts merely the part of an erecting eye- 

 piece." 



There is no answer to this argument so far as it 

 goes. But it really misses the crucial point, the only 

 point of interest, altogether. And if we concentrate 

 our attention on this, the question to be answered 

 becomes this : Does the light received by the 

 microscope from a perfectly corrected projecting 

 lens differ in any essential respect from that which 

 it would receive from a real object similar in every 

 respect to the large one actually used, but dimin- 

 ished in size according to its distance from, and 

 according to the focal length of, the projecting 

 system? 



The only answer to this question seems to me 

 to be that there is no essential difference, and 

 that Lister's results are perfectly valid. For on 

 the older theory, which assumes that objects may 

 be treated as if they were self-luminous, each 

 point in the real object will send out spherical 

 waves towards the projecting system, which turns 

 them into perfectly spherical waves converging 

 towards the conjugate point of the aerial image, 

 from which they expand again so as to form, 

 at a little distance, perfectly spherical waves from 

 the same conjugate point as a centre, precisely 

 as if the conjugate point itself were the true 

 origin. The complicated interference phenomena 

 which arise close to the focal plane, in what the 

 late Dr. Johnstone-Stoney so aptly called the region 

 of turmoil, have no effect on the form of the waves 

 beyond that region. And if we adopt the i\bbe 

 theory we are led to substantially the same con- 

 clusion, for by the combined effects of the principle 

 of equal optical paths between conjugate points 

 and of the optical sine-condition we can easily 

 show that the diffracted waves received by the 

 projecting system are turned into such direction? 

 as to correspond exactly in every respect to those 

 which would be sent out by an actual object of 

 the size and structure of the ideal image of the 

 real object. 



We may indeed say that Lister not only gained 

 the advantage of absolute homogeneity by his 

 procedure, but that he avoided a very grave 

 objection which, in fact, renders open to doubt, 



