nobekt's tests. 199 



true, a table of wave lengths for tlie different colours may be cal- 

 culated, and tlie wave lengths thus deduced agree substantially 

 with those computed by other means. Nobert has discussed the 

 whole subject in two elaborate papers in the 58th volume of Pog- 

 gendorff's Annalen (1852), 1o which I would refer any of your 

 members who are interested in the mathematical asptects of the 

 question. His discussion leaves, as I think, no room for the pos- 

 sibility of a doubt of the objective reality of the lines up to the 

 fifteenth band. 



Now I call attention to the fact that this reason is altogether 

 independent of our ability to resolve the lines with the microscope. 

 In fact, it enabled Nobert to know that his plates were correctly 

 ruled long before the resolution of any but the coarsest bands had 

 been effected by anyone; so that all that Mr. Webb's paper proves 

 is that he does not know how Nobert produces the results, and 

 notwithstanding his great skill in writing on glass, cannot do the 

 same thing himself. 



As no spectral colour is obtained in the bands finer than the 

 fifteenth, the formula of Fraunhofer cannot be applied to them. In 

 fact, the formula demonstrates that if these bands are actually 

 ruled, as claimed, they can give no spectral colour. For my own 

 part, however, I have no hesitation in expressing the opinion that 

 the four higher bands (16th, 17th, 18th, and 19th) have also an 

 objective reality, I base this opinion upon the comparison of their 

 optical appearances as seen with the best glasses with the appearances 

 of the lower bands (especially those from the 9th to the loth). 

 These appearances are quite the same in both cases, and as similar 

 results follow similar causes, I infer the existence of real lines in 

 the four higher bands, since I know beyond the possil>ility of a 

 doubt that they exist in the others. I have discussed the appearances 

 referred to, and the whole matter of the spurious lines which are 

 observed under certain circumstances in connection with the true 

 lines, or instead of them, in the " Monthly Microscopical Journal" 

 for May, 1871. Mr. Webb imagines the real lines also to be 

 spurious, speaks of them as *' aerial polarized black lines of light " 

 (whatever they may mean), and talks generally of the part he 

 supposes polarized lightto play in the production of the phenomena, 

 in a way which shows his optical notions to be original rather than 

 sound. It is hardly worth while to discuss this part of his paper. 



I may mention here, as a matter of interest to the club, that I 



