28 On the Use of the NoheH's Plate. 



high angle may have exquisite definition combined with such cur- 

 vature of field that a part only of any given band may be resolved 

 at a time ; and secondly, that in the case of the higher bands at 

 least, a count of the whole band from edge to edge is so difficult as 

 to be almost impracticable unless special costly and troublesome 

 apj)aratus is employed. 



The first of these objections falls to the ground if the actual width 

 of the bauds is considered in connection with the aperture of the ob- 

 jectives employed. It is asked, " If Nobert had covered a whole inch 

 with the 112,000 and some odd lines, would anyone claim that all 

 must be seen at once ? " Now, the fact is that each of the bands on 

 the plate is really only about the o oW^h of an inch in width ; and 

 the question is not whether an imaginary band of greater width 

 could aU be resolved at once, but simply whether the modern objec- 

 tives as actually made have a field sufiiciently flat to resolve from 

 edge to edge a series of lines occupying a space the two thousandth 

 part of an inch wide in breadth. I have akeady expressed my 

 opinion on this matter, but desire here to ofier a few considerations 

 in its support. 



My Powell and Lealand's immersion sixteenth, with the short 

 eye-piece I generally employ on the plate, gives a field "004 of an 

 inch in diameter, or eight times the width of one of the bands. 

 The ToUes' xVth, belonging to the Museum, with the same tube 

 and same eye-piece, gives a field "008 of an inch in diameter, or 

 sixteen times the width of a band. The Tolles' ith , belonging to 

 the Museum, under the same chcumstances, gives a field ■ 017 of an 

 inch in diameter, or thirty-four times the width of a band. 



With such an eye-piece only the central portion of the actual 

 aperture of the objective is utilized ; and I find that with the fifteenth 

 band sharply in focus at one side of the field I get at the same 

 time complete resolution of the fourteenth and thirteenth bands 

 with both Powell and Lealand's immersion ^Vth and Tolles' iV^h. 

 I cannot, therefore, admit that the actual curvature of field is such 

 as to prevent any given band from being resolved from edge to 

 edge by an objective capable of resolving any part of it, and pass on 

 to consider next the question of the difficulty of a count. 



I have pubhshed elsewhere* what appeared to me to be a very 

 easy and simple method of counting the lines. The circumstance 

 that no one appears to have adopted it, on account probably of its 

 requiring some special apparatus, induces me to mention some still 

 simpler methods, which I have frequently employed with success. 

 If after resolution is attained, a cobweb micrometer be substituted 

 for the ordinary eye-piece, the well-known difficulties resulting 

 from tremor will be encountered if any attempt be made to turn 

 the screw and move the cobweb from line to fine, as is ordinarily 

 * ' Quarterly Journal of Microscopical Science,' October, 1868. 



