
208 THE NORTHERN MICROSCOPIST. 

foci. A disregard of these natural restrictions in the use of wide apertures is 
obviously the origin of the opinion that aperture fer se is antagonistic to good 
definition. It is quite true that there are many even very delicate objects which 
are much better seen under a given amplification with a system of very moderate 
than with one of very wide aperture, the former giving a clear view of the whole 
structure, the latter showing perhaps some distinct points, but as a whole veiled 
in haze. Provided, of course, that we have well-corrected objectives, the fault 
here is not on the part of the lens, but on the side of the object, which requires 
for proper recognition a greater range of depth than is reconcilable with a wide 
aperture. The theoretical suggestion which has been brought forward in support 
of the notion that different parts of the clear area of an objective produce dis- 
‘similar images, and that therefore the resultant image must show increasing con- 
fusion with increasing aperture, cannot apply to the delineation of a plane object. 
In a well-corrected objective the partial pictures received through the various 
parts of the aperture-area are always strictly similar so far as one plane of the 
object is concerned. The confusion suggested is nothing else but confusion of 
the images of different depths—lack of penetration, but not lack of ‘‘definition ” 
in any reasonable sense of that term. Provided the objectives are properly 
corrected and the objects are fit for the delineation of an image, undisturbed by 
interfering confused images from other planes, the “defining power’? of an 
objective is always greater with greater aperture for every kind of objects, inas- 
much as under all circumstances the wider aperture admits of the utilization of 
higher amplification than can be obtained without perceptible loss of sharpness 
(with the same objects) by lower apertures. 
There is therefore no drawback in principle to the use of a large aperture when 
the objects are suitable. But the considerations above lead to the conclusion :— 
Wide apertures (together with high powers) for those preparations 
only which do not require perceptible depth of vision, z.e. for exceedingly 
flat or thin objects, and for transparent objects which can be studied by 
optical sections. Moderate and low apertures when a wide range of 
penetration cannot be dispensed with. 
4. There is still another point of view, and one of special practical importance, 
which shows the positive damage connected with the use of unnecessarily wide 
apertures. The increase of aperture is prejudicial to the ease and convenience 
of microscopical work in two essential respects, 
Istly, It necessitates a progressive reduction of the working distance of the 
objective. Owing to the rapid increase of the anterior aberration with increasing 
obliquity of the marginal rays (particularly in the case of dry lenses), perfect 
correction of a system cannot be obtained unless the layer of low refraction 
between the object and the front lens (¢.e. the working distance) is reduced to.a 
certain fraction of the focal length of the system, which fraction is necessarily 
diminished in a rapid proportion as the aperture becomes greater and greater. 
Whilst there is no objection to retaining as working distance 7-10 of the focal 
length for an aperture of 30°, if the aperture is 60° not more than 3-10 can be 
allowed, and with an aperture of 116° really good correction is not reconcilable 
with a working distance exceeding 1-10 of the focal length. It is therefore an 
obvious disadvantage to use aperture angles of 60° and of 116°, when the power 
which is required or available can be obtained with 30° and 60° respectively. 
2ndly, Increase of aperture is inseparable from a rapid increase of sensibility 
of the objectives for slight deviations from the conditions of perfect correction. 
The state of correction of an objective depends on the thickness of the refracting 
film between the radiant and the front lens, represented by the cover-glass and 
that portion of the preparation which is above the acttial focus. This is a 
variable element independent of the objective itself. In order to avoid large 
