

RELATION OF APERTURE AND POWER IN THE MICROSCOPE, 209 

aberrations which must result from the change of that element, its variation 
must either be confined to narrow limits or must be compensated for by a 
corresponding change in the objective. Now there is a great difference in 
regard to this requirement between the objectives of low and of wide aperture, 
in particular with the dry system. An objective of a few degrees is almost in- 
sensible, it may be focussed to the bottom of a trough of water without any loss 
of performance. With 30° differences in the cover-glasses within the usual 
limits are still inappreciable, and an object may be seen at the depth of a drop 
hanging on the under surface of a cover-glass. With 60° a deviation of the 
cover-glass from its standard thickness by not more than 0.1 mm., or a corres- 
ponding increase of the depth of the preparation above the actual focus, will in- 
troduce perceptible aberrations and a visible loss of definition if not compensated 
for. With an aperture exceeding 100° in a dry lens, the same result will arise 
from a change of thickness of 0.02 mm. only. To preserve always the best cor- 
rection in such a system would necessitate a change of the correction-collar for 
almost every change of focus in the inspection of successive layers, unless the 
preparation is exceedingly thin. 
So far as the necessity of obtaining a certain amount of amplification in an 
efficacious manner requires a certain aperture, the above-mentioned restrictions 
and difficulties in the proper management of the objectives cannot be avoided. 
But all restrictions in regard to the objects, and all the trouble taken in the 
adjustment of the objectives, is quite for nothing when the small result can be 
obtained with a lower aperture. If for the sake of convenience the precautions 
required in the use of wide-angled lenses should be disregarded in working with 
the lower powers of wide aperture, the performance of such lenses is always 
worse than that of much narrower apertures under the same amplification. The 
best wide-angled system, if not carefully adjusted when in use, is not better 
than a bad low-angled lens, for the tolerably sharp image, which could be still 
obtained through the central part of the aperture alone (even under the imperfect 
state of correction) is disturbed by the coarse dissipation of light from the 
ineffective marginal parts of the aperture. 
The amateur who likes the Microscope for his amusement may not much 
object to some extra trouble connected with the use of wide-angled low-power 
lenses, which he admires as brilliant specimens of optical art. For those, how- 
ever, who work with the Microscope, the economy of labour to which they are 
obliged will be expressed by the rule :— 
Never use wider apertures than are necessary for the effectiveness of 
the power, because excess of aperture is always waste of time and labour. 
s. A few remarks about another point of practical interest. By those who 
plead in favour of Jarge apertures in all cases, it has been sometimes suggested 
as a rational plan for reconciling opposite demands, to have all objectives con- 
structed with relatively wide angles, and to reduce them by stops or diaphragms 
when smaller angles are desired. The greater penetration and insensibility of 
the low apertures may of course be attained thereby: but nevertheless this 
device is only a makeshift, and the result is inferior to that obtained by objectives 
originally arranged for a lower aperture. It is not merely that the stops cannot 
increase the working distance (which will always remain at the point corres- 
ponding to the full aperture of the lens), but that the low-angled lens which is 
made out of a good wide-angled one by means of a stop, is in optical respects a 
relatively bad objective—not nearly as well corrected as the same power would 
be if carefully adjusted for the lower angle. The reason will be readily under- 
stood from the following consideration. 
The best correction of an objective of given aperture depends on the proper 
distribution of a certain amount of residuary aberration, which cannot be 
eliminated with our present means. The greater the aperture the more aberra- 
