Microscopical Researches in High Power Definition. 269 



regard to the angular aperture of the most effective kind, whether 

 modified by the limitation of illuminating rays or by reduction of 

 aperture ; or by the refractive media concerned, viz., immersion fluids 

 and "mountings." (Fig. 10). 



Yery striking examples of the disappearance of distinctive shadows, 

 and consequent obliteration of structural molecules, are afforded by 

 the coarser Podura scales ; principally due to the use of excessive 

 angular aperture (figs. 3, 5). 



It is now eighteen years since a single observation suggested the 

 present research, which has been followed up almost continuously 

 towards the attainment of transcendent high power definition. 



Dark molecules suddenly started into view under accidental mani- 

 pulation, but were most difficult of reproduction, and in finer objects 

 of the same kind were often utterly unattainable. 



The questions naturally arose — are certain optical zones in the 

 objectives more effective ; the spherical aberrations existing there as a 

 minimum ; or are there other occult causes of occasional yet splendid 

 definition under high powers ? This did not appear at all to be a 

 question of the minimum visible because the objects exceeded the 

 1-50, 000th of an inch in diameter. 



These points may be illustrated by the records of some observa- 

 tions made by the writer. 



Although the molecules of the scale from the insect Podura 

 domestica are large, the 1-45, 000th, a stringlet of these baffles the 

 powers of the finest glasses now extant, as ordinarily employed, whilst 

 those of the finer test scales (the Podura curvicollis) are hopelessly 

 attempted by every observer who trusts to excessive angular aper- 

 6ure {Pod. domestica, fig. 3, Plate 3) .* 



Example 6. — Data for Resolution. 2-3rd objective as condenser : 

 a U-shaped stop aperture placed on its front lens.f Powell and 

 Lealand's l-50th objective. Direct light of petroleum lamp. 



Result. — A grand display of long rows of whitish molecules in con- 

 tinuous contact (power 2,500), fig. 4, and between these rows are 

 seen closely packed rouleaux of a dark lead-blue colour in a parallel 

 higher plane. The molecules appear like a pearl necklace, in 

 stringlets of twenties and thirties, which can be easily counted, as 

 each molecule appears to be about three degrees of visual angle in 

 diameter. J 



Remove the U-shaped stop, and the whole beautiful resolution dis- 

 appears (fig. 5, left half). If, now, instead of using this difficulb 



* See Plates 73, 74. " Monograph of the Collembola and Thysanura," by Sir 

 John Lubbock, Bart. (Ray Society). 



f This method causes a slight obliquity of illumination, besides considerably 

 reducing the angular aperture of the objective. Aperture of condenser 30°. 



X S = 4irJoo- 10 x 2500=^=,3° nearly. 



VOL. XXXI. U 



