INVERTEBRATA, CRTPTOGAMIA, MICROSCOPY, ETC. 



775 



to follow it u]}. The use of oil as an immersion fluid would obviously 

 have seemed at that time to bo a disadvantage so far as aperture was 

 concerned, in consequence of the diminution of aitgle which was 

 necessarily caused. 



When, however, the bearing of Professor Abbe's theory was 

 appreciated, it was seen that an object-glass acting in oil might take up 

 diifraction pencils which one of larger angle acting in air could not 

 reach, and hence, although the angle was reduced by the use of oil, 

 yet the diffraction pencils belonging to an aperture of more than 180° 

 in air would be compressed (so to say) within the lesser angle, and 

 greatly increased apertures could be utilized. 



" Homogeneous immersion " is thus seen to be essentially depend- 

 ent upon the principles enunciated by Professor Abbe in 1874, 

 and the reason why it was not previously discovered, even by those 

 whose minds were directed to the subject, is explained.* 



Hamilton Smith's " Universal Apertometer."t— Prof. Hamilton 

 L. Smith describes this apparatus devised by him (Fig. 7), which he 

 uses for measuring the true angle in all cases, the old system being, 

 he considers, all wrong, telling a false story in either case, dry or 

 immersion. For angles in glass, or for immersions, the new 

 apparatus may be used precisely like Dr. Abbe's apertometer, and 

 indeed, as it seems to him, has some advantages over that instrument, 

 which will not give the direct air angle, but deduces it from the angle 



Fig. 7. 



in glass ; a separate graduation being required when it is to be read 

 off directly. The 180° are compressed into an arc of 82°, and the 

 whole space on that arc between 60° and 80° is not more than that 

 between 0° and 10°, i. e. the graduations are necessarily unequal, and 

 the instrument is only graduated to every fifth degree. The cylin- 

 drical surface, though it may show a sliding edge with sufficient 

 clearness, is not so good as the more easily made spherical surface 



* Mr. Stephenson draws our attention to an error in his note on p. 490, in 

 which he says that the present homogeneous system " gives an angle greatly 

 in excess of even the ideal maximum of a dry lens (180°) " — for " angle " should 

 bo read " aperture." 



t ' Am. Quart. Micr. Journ.,' i. (1879) p. 194. 



