1909.] NATURAL SCIENCES OF PHILADELPHIA. 191 



On glancing down the tube at back of objective when diatom was 

 resolved, this result was at once accounted for, as only the blue and 

 violet of the diffracted beam, with the merest tint of green, was ad- 

 mitted, and it therefore became apparent that for an objective as 

 perfectly corrected as this one, we must use the column calculated for 

 the "F" line, and not that for the "E" line, which is generally accepted 

 as more nearly representing the effect of white light. It is hardly 

 necessary to add that the resolution was effected with light from an 

 ordinary microscope lamp, without color screen or other aid, as I never 

 test an objective under any conditions, as to illumination or mounting 

 medium, that will differ from those under which it will be used in 

 routine work. It will be noted that the performance of this objective, 

 when correctly interpreted, is strongly confirmatory of the Abbe 

 theory. 



Before proceeding to draw a conclusion from these heterogeneous 

 and rambling notes, collated from records of work at odd times during 

 past couple of }^ears, I wish to reiterate the impartial attitude assumed 

 in undertaking consideration of the subject. If there was any preju- 

 dice, it was in favor of at least a partial acceptance of the dioptric 

 theory. When Wright's book was received and read, although recog- 

 nizing many of its fallacies and rendered suspicious by its commen- 

 dation of such old and discredited devices as the tandem microscope, 

 better known here as "megamicroscope," and the insertion of a stop 

 in the axis of the objective or Ramsden disk of the ocular to produce 

 dark ground illumination, which equals the Abbe diffraction appara- 

 tus in its ability to conjure up "optical nightmares," I was neverthe- 

 less profoundly impressed, and after re-reading it several times and 

 performing some of the experiments with which I had been previously 

 unacquainted, concluded that the Abbe theory must undergo at least 

 some modification; and if I no longer hold that view, it is principally 

 as the result of experiments which were inaugurated with the idea 

 of demonstrating exactly the reverse of the conclusions I was com- 

 pelled to draw from them. 



Much will of course depend on just what is understood to be included 

 in Abbe's theory, on which various writers are by no means agreed. 

 In his own papers Abbe appears to assume a thorough acquaintance 

 with optical science on the part of his readers that few of us possess, 

 hence he makes little reference to known facts and theories, but 

 occasional passages show that he did not neglect to give them full 

 consideration. In the start he unquestionably recognized the effect 

 of absorption and refraction in producing the image, and his theory 



