230 THE MICROSCOPICAL NEWS, 
recognize a decided falling off in sharpness and definition in the 
images in comparing two objectives of equal aperture under the 
same total power, when that power is obtained with one objective 
(of shorter focal length) by a fourfold super-amplification, and with 
the other (of longer focal length) by a perceptibly higher one, ¢.g., 
sixfold ; and that the advantage will always be found on the part of 
the lower ocular. power—whilst no advantage will be gained when 
the same power is obtained with a still more diminished value of v 
(less than 4). It being of course always understood that objectives 
of equal and best attainable construction are compared on sensitive 
objects, and that only the central portion of the field of vision is 
considered. 6 
For example, if a total amplification of 480 diameters is abtained 
in one case with an objective of 1-12th in. focal length, and in 
another case with an equally good 1-8th of the same aperture—the 
figures of v being now 4 and 6 respectively—my view is that no 
practical microscopist would hesitate to declare the image of the 
1-12th to be the Jeffery image ; provided suitable preparations (of 
complicated structure) be observed ; though zo¢ probably on simply 
lined objects and perhaps not on diatom-markings of any kind. 
On the other hand, no decided advantage of any kind will be 
recognized if, instead of the 1-12th, a 1-18th or 1-24th of equal 
aperture is used for obtaining the same power of 480 diameters, 
with of course lower ocular powers. 
Hence it appears that the inherent dissipation of the rays 
arising from technical defects and residuary aberrations remains, in 
carefully finished wide-angled lenses of the dry and the water 
immersion-system, below the threshold of distinct vision .as long as 
it is not enlarged by more than four times, but it is elevated 
beyond that threshold with every greater enlargement. That 
my observations do not indicate a decided difference between dry 
and water-immersion lenses may be well accounted for by the fact 
that the advantage of diminished front-aberrations in the immersion 
system is balanced by the increased aperture. With immersion- 
lenses of not more than 1‘o or less (other circumstances being 
equal), a somewhat higher value of vy would be found. On the 
other hand, I have always observed a perceptible lowering of the 
critical super-amplification with objectives of greater apertures than 
ogo for the dry system, and of 1°20 for the water-immersion, 
when preparations are used for the experiment which put the 
utmost marginal zone of the aperture in action szmu/¢ancously with 
the intermediate portions between the centre and the margin. 
(2) A decided advance in the performance, in regard to the 
critical value of v, is found in well-made objectives of the homo- 
geneous-immersion system. With the same standard of judgment, 
and on the same principle which has been explained above, I con- 
