Messrs. Sullivant 

 Analysis of 



and Wormley on Nobert's Test Plate. 209 

 Nobert's Test Plate of SO Bands. 



The figures in the 3rd and 6th columns^ showing the dis- 

 tance apart of the hues in each band, are the mean of numerous 

 and sHghtly variant trials^ particularly on the higher bands. 

 Up to the 26th band there was no serious difficulty in resolving 

 and ascertaining the position of the lines ; but on this and the 

 subsequent ones, spectral lines* (that is, lines each composed of 

 two or more real lines) more or less prevailed, showing that the 

 resolving power of the objective was approaching its limit. By 

 a suitable arrangement, however, of the illumination, these 

 spurious lines were separated into the ultimate ones on the whole 

 of the 26th, and very nearly on the whole of the 27th band ; 

 but on the 28th, and still more on the 29th, they so prevailed, 

 that at no one focal adjustment could more than a portion (a 



* The tendency of lines near the limit, either way, of the objective's 

 resolving power, to run into each other and produce spectral or spurious 

 lines, is readily shown by a low objective on the lower bands. Hence the 

 mere exhibition of lines is not always conclusive evidence of their ultimate 

 resolution. A practised eye will generally distinguish the false from the 

 true. Recourse to a higher objective often accomplishes the same ; but 

 when these fail, the micrometer only, together with a previous kuowledo-e 

 of the actual position of the true lines, can determine whether the lines 

 exhibited are real or spurious. A -j'.^ or j\ will show the three or four 

 highest bands on this plate regularly and beautifully striped with lines much 

 coarser than the true ones ; the same with the yV on the last band. 



