SUPPLEMENT. 



395 



sir, that in my Dunkirk lecture I did use the words " plus 180 

 of aperture/' Now that select and intelligent audience (com- 

 prising physicists who did know something about " balsam 

 angles,") so far from demanding an " apology," generously and 

 unanimously gave me a vote of thanks. Curious, wasn't it ? I 

 remember well my delight in there meeting the veteran Dr. J. 

 W. Armstrong, Principal of the Eredonia Normal School, and 

 one of the leading educators of your state; another physicist, 

 too, who can talk intelligently as to " balsam angles," and who- 

 has made the apertures of objectives an especial study, and 

 who afterwards became one of my most valued correspondents. 

 Nor did we have any quarrel about the "plus 180 Q ." Most of 

 the audience had read about Joshua, and doubtless would not 

 baulk at such expressions as " sunrise '' or u sunset." 



After all, professor, admitting that the " plus 180 " might 

 have courted son..e such gentlemanly criticism as it finally got 

 from you, I reckon I was as near right as Joshua. But before I 

 can lift you over this stile, I must scoop you up ! get you 

 together in some shape, so you can be handled. First of all, 

 you seem to put the 180, plus 180, 180 X 45, all in one boat 

 together; you regard them as synonyms. And, secondly, I 

 have to learn some things from you. 



Messrs. Tolles and Spencer you denounce because they mark 

 their objectives 180, which you say is not only " impossible,"" 

 but " absurd," and that one of these gentlemen (you don't say 

 which) attempts to lead the scientist " by the nose." And you 

 further say that one ot them (I don't know which, again,) 

 " might as well add 45 to said 180. 



Well, here is material enough to commence on. Now, my 

 dear professor, you maintain it to be the " right and duty " even 

 of every man of science to ask this little question whenever it 

 occurs to him the " why" part you know). That I have been 

 engaged in a controversy with you for months, demonstrates 

 my claim as a " man of science." You can't dodge that, and 

 the "occur " part is present and up to the high-pressure notch. 



Now, I want to know " why " it is that 180 of aperture is 

 impossible ; "why " that plus 180 of aperture is impossible and 

 absurd. We have your assertions, but minus the ghost of a dem- 

 onstration. 



