SUPPLEMENT. 395 



sir, that in my Dunkirk lecture I did use the words " plus ISO 01 

 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 Fredonia 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 " 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 r 

 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, ' r 

 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, > 

 " miff lit 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 i 

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

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

 onstration. 



