804 Transactions of the Society. 



ferent objects (artificial preparations and natural objects), and 

 checking my own observations by the judgments of some ex- 

 perienced working microscopists in the department of Biology. 

 "What I consider as the outcome of that systematic trial will be 

 briefly indicated here, with all that reserve which is necessitated 

 by the nature of the problem. The principal points are : — 



(1 ) With the best ivide-aperture objectives which have been made 

 anywhere up to the present time (1882), dry or water-immersion, of 

 apertures not less than " SO and 1 • 10 respectively, the deterioration 

 ot the image by the manifestation of aberration-effects becomes visible 

 as soon as the super-amphfication {v) in use is greater than about 

 4 times ; * that is to say, that any trained observer would 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, e. g. sixfold ; 

 and that the advantage icill always he found on the ixirt 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 ob- 

 jectives of equal and best attainable construction are compared 

 on sensitive objects, and that only the central portion of the field 

 of vision is considered. 



For example, if a total amplification of 480 diameters is obtained 

 in one case with an objective of 1-1 2th in. focal length, and in 

 another case with an equally good l-8th of the same aperture — the 

 figures of V being now 4 and 6 respectively — my view is that no 

 practical microscupist would hesitate to declare the image of the 

 l-12th to be the better image; provided suitable preparations (of 

 complicated structure) be observed; though not 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 l-12th, a l-18th or l-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 mi)re than four times, but it is elevated 

 beyond that threshold with every greater enlargement. That 



* I leave out of consideration here some particular objectives (recorded in tliig 

 Journal, vol. ii. (1879) p. 815) which were made some years ago bj- C. ZvUs, for 

 experimental purposes, on a system of construction which is not applicable to, 

 autl was not intended for, regular use. 



