1904.] natural sciences of philadelphia. 475 



May 3. 



]\Ir. Arthur Erwin Brown, Vice-President, in the Chair. 



Thirty-nine persons present. 



A Spencer Objective. — Mr. F. J. Keeley described a microscope 

 objective of one-fourth inch focus, made in 1860 by Charles A. Spencer. 

 It was recently necessary to take apart the back system for re-balsam- 

 ing, Avhen it was fomid to consist of five lenses, three of which were 

 convex and two concave. One of these proved, on examination with 

 polarized light, to be fliiorite, which mineral, while isotropic, exhibits 

 characteristic optical anomalies between crossed prisms that permit 

 of its identification, at least in contradistinction from glass or other 

 material used for lenses. The fluorite lens is perfectly preserved, as 

 might be expected from the stable character of the mineral. When 

 objectives of more recent manufacture containing fluorite have deteri- 

 orated, the fluorite has been blamed for faults which should imdoubtedly 

 have been attributed to unreliable glass used in connection with it. 



This objective is historically interesting as illustrating the complex 

 nature of the corrections adopted by Spencer at so early a date, as 

 well as confirming the previous reports that he had appreciated the 

 possibilities connected with the use of fluorite in securing superior 

 color corrections and employed it for the purpose twenty years before 

 it came into use abroad. 



The objective has an aperture of 142 to 152 degrees, according to 

 position of adjustment, which acts by rectilinear movement of back 

 systems, and is unusualh^ well corrected for color. It resolves Pleuro- 

 sigma angulatum sharply into dots with central light from mirror, 

 and with oblique illumination resolves markings 76,000 to the inch. 

 In some respects its performance was possibly slightly sacrificed in 

 eHminating color, for with a large central illuminating cone, its defi- 

 nition is somewhat inferior to that of objectives of similar power made 

 by Tolles at slightly earlier and later dates, which show considerably 

 more color. The latter, although both over forty years old, compare 

 favorably, optically and mechanically, with best achromatics made to- 

 day, and it seems not unlikely that Spencer abandoned the use of 

 fluorite because he realized that sharp definition was more important 

 than the elimination of last trace of color, rather than from any fear 

 of its lack of permanency. 



The Publication Committee reported that a paper had been presented 

 for publication under the title "Observation on Hyrax," by Henry 

 C. Chapman, M.D. (April 29, 1904). 



The following was ordered to be printed : 



