On the Use of the Noherfs Plate. 31 



right and the coarse ones on the left, it will be found that on very 

 slightly withdrawing the objective from the plate the fine and coarse 

 false lines change place almost exactly as though the direction of the 

 light was changed. This change is not a sudden one ; fine spurious 

 hues begin to be seen on the second side before they have all dis- 

 appeared from the first, and there is no intermediate position such 

 as may be attained with the lower bands, in which there are abso- 

 lutely no spurious lines to be seen on the edges. 



These results of a change of focus enable the observer to dis- 

 tinguish in the microscope the position of the last real hne on either 

 side, and thus to attain accuracy in his count ; and if a successful 

 photograph be compared with the object as seen in the microscope, 

 there wiU be no great difficulty in determining, first on one side 

 and then on the other, the portions of the picture which correspond 

 to the true hmits of the band. It was in this way that I deter- 

 mined the hmits to be given to the enlargement of my photograph 

 of the nineteenth band, which was distributed in every case pasted 

 on the same card with a print from the unmodified original negative. 

 The object of the hmitation given to the enlargement was to make 

 it serve as a representation of conclusions, readily obtained in the 

 microscope by change of focus, but which the possessors of the pho- 

 tograph could not arrive at without repeating my observations, 

 since the photograph represented of course but a single focal posi- 

 tion. 



If the first and last real hnes are fixed in such a photograph by 

 comparing it with the appearances in the microscope, the photo- 

 graph will answer a useful purpose in verifying the count. Even 

 if the observer is not sure as to this, if he can find, as he generally 

 can, any reference points in the microscope and in the photograph 

 by which fixed points in the band can be identified, a count of the 

 intermediate lines will serve as a useful check. I have made use of 

 all such aids in my study of the Nobert's plate, but I desire to say 

 expressly that my statement of the number of lines in each band 

 rests essentially on my counts in the microscope, and that I have 

 repeatedly counted in the microscope the hnes of each of the bands, 

 from edge to edge. I may also make a single remark in this place 

 with regard to our knowledge of the number of lines in the bands. 

 I believe I was the first to state the actual number for the higher 

 bands of the new nineteen-band plate. Nobert gave the distance of 

 the lines apart in fractions of a Paris Hne, the actual width of the 

 bands and the number of hnes in each was not stated. From his 

 figures the number of lines per millimetre and per inch has been 

 computed. But the practical difficulty of measuring the width of a 

 band with sufficient precision to deduce the number of lines from 

 these figures has caused aU the writers on the subject, whose papers 

 I have seen, to observe a judicious reticence as to the actual number 



VOL. VI. D 



