30 On the Use of the Noherfs Plate. 



be found too few ; for instance, twenty to forty in the nineteenth 

 band, instead of fifty-seven. The laws governing the production of 

 these false lines and the relation of their number to the aperture 

 of the objective and the obliquity of the illuminating pencil, have 

 perplexed the most eminent students of optics, and are well worthy 

 of future investigation. It is enough for my present purpose to 

 state tliat while false lines of this character may be seen in the place 

 of the true hues of the higher bands with objectives perfectly 

 capable of resolving them with more obhque illuminating pencils, 

 a similar appearance is often the best that can be produced with the 

 most oblique pencils, if the resolving power of the objective is in- 

 adequate. These thin spurious lines differ in several particulars 

 from the real ones of the higher bands. The latter are not smooth, 

 they are irregular, they are thicker than the interspaces, they are 

 wavy. The cutting tool has moved with a certain tremor ; some of 

 the lines are ploughed deeper than others, the distances from centre 

 to centre are not always equal. Such inequalities might be ex- 

 pected in ruling such fine lines, and with adequate defining power 

 they will readily be recognized. 



B. Sjjurious Lines seen on the Margins of the Resolved Bands. 

 — These require careful consideration by those who attemj)t to count 

 the higher bands ; but by selecting for study at first a moderately 

 fine band, say the eleventh or twelfth, their characters can be 

 learned, and the method of distinguishing them from the true ones 

 mastered. 



Suppose, for example, the twelfth band is under observation ; 

 when the pencil is sufficiently oblique to show the true lines, a series 

 of fine spurious lines, closely resembling true ones, are seen adjoin- 

 ing the band, which, when the objective is in the lowest focal posi- 

 tion compatible with definition, appear on the side from which the 

 oblique light comes (as the microscope reverses, they are of course 

 really on the opposite side). Of these spurious lines those next 

 adjoining the real ones measure from centre to centre the same as 

 the real ones, but the more distant ones grow gradually fainter and 

 more separated till they disappear from view. On the opposite side 

 of the band are a few coarser spurious lines, easily distinguished 

 from the real ones. Such is the condition of things shown in my 

 photograph of the nineteenth band. I do not think anyone could 

 tell by mere insj)ection either of the photograph or of the image in 

 the microscope, if the focal adjustment remains unaltered, which 

 was the last true line, and which the first spurious one, on the side 

 of the fine spurious lines ; on the other side it is easy enough. But 

 if now the direction of the light is reversed, the fine and coarse 

 spurious lines change places, and what is still more important, a 

 similar change can be effected by a mere change of focus. In fact, 

 when the band is seen as above, if the fine spurious lines are on the 



