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Nl.—On the Use of the Noherfs Plate. 



By Assistant-Surgeon J. J. Woodwabd, U. S. Army. 



I DO not think the question of priority as to the resolution of the 

 nineteenth hand of the Nohert's plate, ahout which Mr. Charles 

 Stodder makes such warm reclamations in the March number of this 

 Journal,* possesses in itself enough general interest to make it worth 

 while for me to add anything to what I have heretofore written on 

 this head. If, however, as seems very probable, the Nobert's plate 

 is to be much employed in ascertaining the comparative defining 

 power of fine objectives, it is important that those who use it should 

 have some reliable means of knowing whether they have actuaUy 

 resolved any given band ; and such loose ideas appear to be enter- 

 tained with regard to the matter in many quarters that it seems not 

 undesirable to ofler a few remarks on the difficulties involved, and 

 the best means of overcoming them. 



The satisfactory resolution of the Nobert's plate is, as is well 

 known, complicated by the readiness with which spurious lines make 

 their appearance parallel to the real ones, and simulating them 

 more or less closely according to the character of the illumination 

 employed. This difficulty is greater with the higher bands than 

 with the lower ones, and with oblique light is more deceptive than 

 with central illumination. 



Three criteria for distinguishing the spurious lines from the true 

 ones have been offered. 



The first is the unaided judgment of the individual microscopist, 

 who is supposed to be able instinctively to distinguish the false from 

 the true lines without any special help. 



The second is the enumeration of the lines in a measured por- 

 tion of the band, and the comparison of the results attained with the 

 statements made by Nobert as to their real distance. 



The third is a count of all the lines in the band supposed to be 

 resolved. 



With regard to the first of these plans I must continue to think 

 that it is utterly untrustworthy, and that should it unfortunately 

 be generally accepted the plate would cease to possess any value as 

 a measure of resolution, for individual enthusiasm would lead many 

 to suppose they had succeeded, when in fact they were provided with 

 utterly inadequate means. 



It may be granted that an observer who has many times efiected 

 the true resolution of any given band will at length have its appearance 

 so firmly impressed upon his mind that he will recognize it when- 

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



* P. 118. 



