20 On the Hiffh-Power Definition of 



angled one of fine qualities ; yet be it ever remembered all tele- 

 scopes are of low angle, however fine their performance. 



A low-angled objective gives more precise details of structure ; 

 it has a deeper focus, and presents a more natural view. The 

 human eye possesses an angular aperture of about one degree. 

 Who can tell what our idea of objects would be if it imitated the 

 modern glory of the diatomist and viewed particles with the tre- 

 mendous aperture of 140 degrees ! Behold the ravishing beauty of 

 innumerable objects viewed vfith a four-inch objective, necessarily 

 of small aperture, and choose between the two kinds. 



Doubtless the extreme difficulty of obtaining sufficient light 

 will always compel microscopists to require a sufficient aperture. 

 But there is another most important aspect, viz. the effect of low 

 aperture on sharpness of definition. 



Take threads of spun. glass, cylinders of glass, and glass globules 

 fused at their ends. I have shown (' Quart. Jour. Mic. Sc.,' Jan. 

 1870) the images produced by the crossing of cyHnders of glass ; 

 and the dependence of the black edge or shadow as regards its 

 breadth upon two conditions. 



(a) The angular aperture of the observing objective. 



(b) The refractive index of the substance itself. 



Now as the aperture is reduced by the aberrameter,* the 

 black ring of a glass bead of ooVoth of an inch in diameter gradu- 

 ally broadens itself until the centre is almost occluded: whilst the 

 black borders of the fine cylinder of glass become broader and 

 broader also. 



Now, when organic particles which appear from their action 

 and reaction, as regards shadows and spurious disks, to be spherical, 

 are gradually treated similarly by a contracting aperture, they also 

 become in many cases more pronounced in a darker outline. It is 

 curious to see the effect upon the spherules of many scales by this 

 very thing. Belief, depth of focus, darkness of outline, in a good 

 glass, all reward the operation. In many cases, structure unsus- 

 pected before, has started into view. 



Moderate angular aperture is therefore greatly to be preferred 

 in all researches on organic structure involving the definition of the 

 minute organic particles of which they are composed, where dark- 

 ness of outhne is a desideratum as well as depth of focal perspective. 

 In fact, some objects are much more difficult of observation with a 

 very high than a much lower angular aperture. 



The structures of the Poclura curvicollis were first seen by me 

 in 1862 with a fine Boss 1851 \ objective and 70^ aperture, 16 

 inches draw-tube, a C eye-piece, and an oblique narrow pencil from 

 a moderator lamp placed on a stand on a level with the table. I 



* Marie for me by Messrs. Beck, and consisting of an iris-diaphragm placed 

 behind tlie objective. 



