SUPPLEMENT. 397 



kirk audience ; when you shall have proven what 97 of balsam 

 angle is ; ditto as to " true aperture," and how to "definitely 

 measure " the same then and riot until then will I take the 

 "plus 180 " under advisement. Meanwhile we will be good 

 friends, and with Gen. Grant say, " Let us have peace." Don't 

 omit reading the reprint of Wenham's article on angular aper- 

 ture, pages 74 and 75. 



CHOICE OF OBJECTIVES. 



Ed. Am. Jour. Microscopy. It was with real pleasure that I read 

 the article entitled " Dr. Carpenter on Angular Aperture," by 

 W. G. Lapham, Esq., in your May issue. 



First of all, it is very satisfactory, from the fact that Mr. 

 Lapham speakes from a practical standpoint. The gentleman is- 

 an entire stranger to me, but the fact that his conclusions are 

 drawn from his personal, and, I doubt not, protracted experi- 

 ence, is evident from the ability with which he handles his- 

 subject. 



Your readers are well aware that I have often complained 

 that I have been compelled to fight theory with practice and 

 that no effort on my part sufficed to induce my opponents to 

 abandon their theoretical ideas, and to examine practically as to 

 the truth of the positions advanced by myself. In Mr. Lap- 

 ham, however, I resognize a co-worker, and althiugh the 

 results he arrives at are not in perfect coincidence with my own, 

 I find nothing to criticize, but am simply desirous of further 

 comparing notes with the gentleman, in the hope that some- 

 thing to the advantage of microscopy may be developed. 



I propose to present a few thoughts for Mr. Lapham's con- 

 sideration, requesting the gentleman to give them whatever 

 study and attention he may elect, and to advise your readers- 

 as to the conclusions he arrives at; and, as before intimated, 

 nothing that I have to say ought to be taken in the light of a 

 criticism of his really excellent paper. 



First, I desire to ask if Mr. Lapham does not attach too much 

 importance to the so-called " penetration." Is there in esse, any 

 such thing as "penetrations?" Perhaps it will be well, first of 

 all, to settle this point. 



