The National Microscopical Congress. 65 



believe that by recommending and adopting a definite meaning for the 

 term we can advance the interests of true science ; and 



Whereas, in the present state of our knowledge of objectives, we 

 see no indication that the capacity of an objective to resolve lined tests 

 depends upon its angular aperture alone, as distinguished from other 

 optical qualities apart from workmanship — 



We therefore adopt the following resolutions : 



Resolved, that we adopt the following definition of angular apert- 

 ure as applied to the objectives used with the microscope : 



The angular aperture of a microscope objective is the angle at the 

 apex of a triangle, having a base equal to the available diameter of the 

 front lens, and a height equal to the actual focal length (working dis- 

 tance), measured in air for a dry lens, and in the fluid employed for an 

 immersion, the collar being adjusted for the most perfect definition in 

 every case. 



Resolved, \hzx.y^^ request all makers to mark their objectives in future 

 to correspond with the definition above adopted. 



Resolved, that this preamble and resolutions be distributed in circular 

 from among the various societies and makers of objectives, with the re- 

 quest that they give it their formal approval, and communicate what- 

 ever action they may take to such body as the Congress may appoint. 



Resolved, that we recognize that the interposition of cover-glass, bal- 

 sam, or any other medium of a different refractive power from the 

 one for which the aperture is given, has more or less effect upon the 

 aperture and image, and we recommend this as as a subject for investi- 

 gation. 



These called forth some remarks from several members, among 

 them, his most prominent opponent, Prof. J. E. Smith. Prof. 

 Smith, however, undertook to argue that the author was not 

 debating the subject of aperture, as pretended, but was merely 

 arguing in favor of low angled glasses as against high angled. 

 This was so obviously wrong that it was flatly denied, and, in 

 order to bring the matter to a focus, and to induce the sup- 

 porters of the terms 180° and -f 180°, to come fairly before 

 the Congress, Mr. Hitchcock read the following challenge : 



It will be admitted by all in this audience that no ray of light can 

 take any course that cannot be shown by a diagram, and calculated 

 mathematically. 



The opposition which meets this paper is not unexpected. In order 

 that my opponents may show themselves in the right, if possible, I offer 

 the following : 



I challenge any man who is ready to champion the side which claims 

 the angular apertures of 180° and-|-i8o° as possible apertures, to step be- 



