226 Dr Royston-Pigott, On a new method, etc. [Dec. 2, 



we see that as the semi-aperture diminishes the limit of visibility 

 of bright spaces between dark lines is rapidly changed. At 60" 

 aperture sin a = sin 30° = |-, e = \ instead of |A, in the limit. 



But it is found that reduction of the aperture of the object- 

 glasses employed in this experiment does not destroy the sharp 

 visibility of the spider-lines ; on the contrary, it improves their 

 sharp blackness and definition : the limit being reached at last by 

 deficiency of light, more pronounced in dry objectives. The supe- 

 riority of immersion lenses over dry ones is veiy evident in the 

 shai-per visibility of these tiny miniatures. The best results are 

 obtained by using a pair of them, a drop of water being inserted 

 between their noses. But for dry lenses, these test lines can be 

 discerned thoroughly well with glasses of all apertures, so long as 

 the observing objective is about half the power of the miniaturing 

 glass, their relative proportions being thus kept up, the actual 

 visual angle 6 will rem±ain the same, and the spider-lines appear 

 of the same delicate tracery as before, notwithstanding the enor- 

 mous reduction of aperture. 



The absolute necessity then for large aperture appears by these 

 experiments to be a delusion, for dark lines, mildly illuminated, 

 ranged exceedingly close together. I possess an excellent ^ triplet 

 by Wray of Highgate, which shews these lines as sharply as can 

 be desired under the same visual diameter, 18 seconds, although 

 the aperture is only 40°. The same thing can be seen with lower 

 poAvers. 



Dr Carpenter, F.R.S., has long contended for the superiority of 

 small aperture glasses for physiological research, and the visibility 

 of objects subtending no greater angle than Nobert's lines in his 

 finest band by such glasses is an amazing proof of the truth of his 

 opinion. 



The extreme obliquity of illumination necessary to develope 

 shadows sufficiently black to represent lines, in the case of a 

 multitude of diatoms, etc., has no doubt originated the extreme 

 angular apertures of some of the most expensive glasses. Tolles 

 in America obtains the very high price for his ^ objective (immer- 

 sion) of 16 guineas, and this glass resolves the diatom Ampliipleura 

 pellucida, shewing 100,000 lines per inch. In these complicated 

 structures catchiug the shadows is the secret of successful reso- 

 lution. But for the most useful investigations excessive aperture 

 is in many cases, in the. writer's opinion, incompetent to deal with 

 minute research. - 



Haktlet Court, 



Nov. 26, 1878. 



