66 Atiuruan Quarttrh Muroscopical Journal. 



fore this audience and demonstrate by diagrams the following proposi- 

 tions : 



1. That light at an angle of i8o" can enter an objective. 



2. That a balsam angle of over S2' can be employed with any objec- 

 tive, without special arrangements for illumination. Unless this can be 

 demonstrated, I maintain that the high balsam angles do not dep>end 

 upon the aperture of the objective but upon the sub-stage accessories.* 



3. That an objective can take in light, theoretically or practically. 

 infinitely near 180' either dry or immersion. 



4. That the terms 180' and -j- i8o\ have any definite scientific mean- 

 ing or rest upon any basis of scientific accuracy. Unless they do. I 

 mj^intain that they should be thrown out of scientific literature. 



5. Whoever may accejit this challenge must agree to reply to such 

 questions as may be asked, and the whole of his argument must be re- 

 ported by a stenographer for the Congress. 



If this challenge is not accepted I shall feel at liberty to ignore the 

 rlaitnt of my Opponents in future. 



This provoked some discussion, but of a rather general char- 

 acter, and. in order to confine it, Mr. Hitchcock asked if any 

 one would accept the challenge. He was asked to read it 

 again, which was done. Dr. Geo. E. Blackham then took the 

 floor and showed on what grounds the figures 180'. and — 180' 

 were upheld, and touched upon some other points. Dr. Black- 

 ham's statement of the case was. so far as it related to the 

 actual course of the rays of light, perfectly fair. 



Prof. J. E. Smith then arose and made a protest against chal- 

 lenges. He considered them undignified. 



The challenge was not accepted by any one. and it should 

 be noted that there were present the most noted supporters of 

 these extreme angles. 



Dr. Ward made a few remarks uf)on the subject, in which he 

 opposed the passage of the resolutions, on the ground that the 

 Congress should not take hasty action in a matter about which 

 there was so much difference of opinion. Mr. Hitchcock 

 stated that his object in offering the resolutions was merely to 

 confine future discussion, by limiting the meaning of the term 

 "angular aperture." He then withdrew them as advised by 

 his friends. 



On Friday the principal subject was micrometry, and as this 

 is a matter which is of considerable importance, and one which 

 will engage the attention of microsccpists for some time to 



*See paper in Am. Jonn. HV , Jme, 13TS, p. 132. 



