SUPPLEMENT. 373 



Dr. Johnson's experiments with the 19th band published V (as 

 were my own). Again, were they " subjected " to public exam- 

 ination ? " ( as were my own.) Let the Professor's rule 

 work both ways. Meanwhile I now assert, and know 

 whereof I affirm, that a Powell & Lealand l-10th of 1872 

 cannot be forced to show the 19th band by lamplight. In gen- 

 eral, I don't propose to accept anything done with the 19th band 

 five years ago, as comparable with what is being done in 1877. 

 Again, all who have seen the 19th band as displayed with the 

 duplex and modified Beck illuminator, instantly admit that for 

 wondrous delicacy of definition, the total absence of the "ghost" 

 of a diffraction line, all other methods of illumination (by arti- 

 ficial light) are simply " nowhere." And further, that any 

 attempt at photographing these wonderful lines has been but 

 approximative. To be more definite, the best photograph of the 

 19th band yet made is simply a caricature. Nevertheless it is 

 wonderful that Col. Woodward has succeeded as well as he ha.s, 

 and his photographs are the best that I have seen. 



A word as to the mathematical tables. There can be no pos- 

 sible objection to these, when (it's my chance to say "when" 

 now) they agree with obvious facts ; they are objectionable when 

 they are not in accord with facts. I stand ready to accept truth 

 in any form ; ditto, to repel error. Now the facts are, that from 

 a mathematical standpoint the limits of vision have been 

 erroneously " set " again and again. I simply protest against 

 fighting facts with mathematics simply this, and nothing more. 



In drawing towards the close of this article, I desire, as apro- 

 pos to the question at issue, to state a little bit of personal expe- 

 rience, viz : About a year ago a gentleman, a professor occupy- 

 ing a responsible position, gave me a carte blanche to provide him 

 with a medium power lens. He was particular to state that he 

 was engaged in histological studies, and wanted the glass for 

 such and similar purposes. In response, he was furnished with 

 a l-6th duplex. Last summer he visited me during his vacation 

 and became my pupil. He was, of course, an apt scholar, and 

 quick to learn. He spent with me weeks in acquiring the man- 

 ipulations of this glass, and became quite expert. When he left 

 me he said, " I have now learned practically what was done for 



