202 CORKESPONDENCE. 



add anything to what he has already written. What he has written is 

 evidence that he then thought it was a matter of gi'eat interest. It is 

 true the question does not, outside of the circle of microscopists, possess 

 the interest that the invasion of France by Prussia did in the political 

 world ; but within that circle it does possess as much interest as does 

 the cession of Alsace to Prussia in the political. 



The first construction of an instrument capable of that feat marks 

 an event, an era in optics, as important and remarkable as the re- 

 nowned improvements of Lister and Andrew Ross.* 



Dr. W. says, three criteria for distinguishing the spurious from 

 the true lines have been offered. He specifies, " The first is the un- 

 aided judgment of the individual microscopist, who is supposed to be 

 able instinctively to distinguish the false from the true lines, without 

 any special help." By whom was such a criterion ever offered ? Cer- 

 tainly not by me. 



On the same page — in the same breath, so to speak — Dr. W. con- 

 tinues : " It may be granted that an observer who has many times effected 

 the true resolution of any given band, will at length have its appear- 

 ance so firmly impressed upon his mind that he will recognize it 

 whenever he sees it, as he would the face of a familiar friend ; but this 

 familiarity, which all acquire with any appearance which they have 

 many times reproduced, will only serve to mislead, if at the beginning 

 spurious lines have been confounded with the ti'ue, for then the decep- 

 tive spui-ious aj)pearance will be sought for as eagerly as though it 

 were the true one." Good ! Dr. W. having granted that, has granted 

 all I ask for. That has been my principle of observation, caution 

 and all. I could recognize the true lines as the face of a familiar 

 friend. 



A few other passages in Dr. W.'s paper require notice. He says 

 the question is " simply whether the modern objectives as actually 

 made have a field sufficiently flat to resolve from edge to edge a series 

 of lines occupying a space of the 2000th part of an inch wide." No 

 question like that has ever been offered or.suggested by anyone before to 

 my knowledge, not even by Dr. Woodward himself. Flatness of field 

 is a desideratum in an objective, but not the sine qua non. The real 

 question is, can an instrument be made of such defining power as to 

 separate or to show the lines or the spaces between two lines ruled 

 to the fineness that the nineteenth band is ? Mr. Huxley said, " Histo- 

 logists, he feared, had come to the end of their work unless .... 

 they could obtain microscopes which would enable them to separate 

 two points the 100,000th part of an inch apart."| Mr. Huxley can 

 scarcely be posted as to what modern microscopes can do. If the lines 

 of the nineteenth band are as wide as the spaces between them, they 

 are only 1 — 224,000th of an inch apart. 



The problem is strictly analogous to that of separating double 

 stars — a matter of definition. If the microscoi:)ist can see the spaces 



* See Carpenter, 'The Microscope and its Eevelations,' 1st ed., 1856, p. 197, 

 Philadelphia, and every edition since to 1868 ; also Dr. H. H. Hagen's Kemarks, 

 ' Proo. Boston Soc. Nat. Hist.,' 1869, vol. xii., p. 359. 



t 'M. M. Journal,' Nov., 1S70, p. 291. 



