530 Transactions of the Society. 



a discovery, should remove the slide and place a Grayson's band- 

 plate upon his stage, and see which band was resolved, without 

 disturbing the illumination or any of the adjustments except the 

 focus. The discovery should be recorded as follows : Structure 

 seen, oil immersion y 1 ^ N.A. 1*3, comp. ocular 12, X 1500, Gifford 

 screen, Grayson band 60,000. A statement such as this clears the 

 announcement of all ambiguity, and it becomes apparent that 

 either his objective, apparatus, or eyesight must have been defective, 

 or else a small cone of illumination was employed. But in the 

 absence of any such additional information, the conclusion one 

 would naturally come to, with the table of limits printed at the 

 end of the Journal before them, would be that the capacity of the 

 investigator's apparatus was about 135,000 to the inch. 



The statement made in my former paper,* to the effect that a 

 screen did not make any difference in the limit with apertures 

 higher than 0*8 N.A., requires correction. I have altered my 

 opinion with regard to the effect of the blue-green screen. The 

 experiments upon which the previous statement was based were 

 performed with a light from a £-in. paraffin wick, but, in order to 

 bring out the effect of a screen, a much more powerful illuminant 

 is required. An ordinary acetylene bicycle lamp will be found an 

 admirable illuminant for high power microscopical work with 

 screens. I believe the photographic limit with a wave-length of 

 • 4 fj, is too high, the true photographic limit being much the 

 same as the screen limit. 



Probably the strongest resolving lens in existence, for its aper- 

 ture, is the Zeiss long tube apochromatic 4 mm. This lens will 

 visually resolve, with a screen, the Amphipleura pellucida, but no 

 one has ever heard or seen a photograph of Grayson's 120,000 

 band taken with this lens, which, according to the • 4 p wave- 

 length photographic table, is within its grip. 



It will be seen that this table differs in form from that at the 

 end of the Journal. First, the three columns of equivalents in 

 angular aperture are omitted. The information that an oil-immer- 

 sion of 40° of aperture, a lens which is not made, and never has 

 been made, is the equivalent of a water-immersion of 46° 2', which 

 is also a lens that has never been made, was thought to be hardly 

 worth the space. In place of these three columns, the number of 

 lines in the ^ of a millimetre are given. The total number of 

 figures has been reduced, by giving the number of lines in the 

 j^-q of an inch, which in no way impairs the usefulness of the table. 



N A 

 W.A. is equal to — '- — ' : this equivalent is used at the head of the 



columns of illuminating and penetrating powers ; these functions 

 of a lens depend upon the amount of aperture utilised, and not upon 



* This Journal, 1893, p. 15. 



