476 



MANIPULATION. 



La Rue, in a test plate received from M. Nobert in January, 

 1850:— 



On examining this plate with a -^-^ object-glass of 140° 

 aperture, Mr. De La Rue found that the lines are seen on all 

 the bands ; but the lines seen on some of them are spectral, 

 and not the real lines ; an effect which may be demonstrated 

 on observing the plate of fifteen series, say with a half-inch 

 object-glass at first, and then one-fourth ; the"haF-inch object- 

 glass will show many of the series to be lined which it fails to 

 develop clearly, but which are readily resolved on using the 

 one-fourth. Having observed this, Mr. De La Rue requested 

 M. Nobert to take note of and give him the number of the lines 

 of each series in all future plates he might make for him, which 

 he accordingly did. In this way he was able to control the 

 observation, and pronounced the eighth series to be perfectly 

 and clearly separated with Ross's -^-^ of 140°; and he 

 thinks that even the -j?^ has yielded to its defining power ; 

 but it is so difficult to count the fine lines, that he cannot 

 speak with absolute certainty, but he has no doubt in his own 

 mind respecting it. M. Nobert has since furnished Mr. De 



