A PLEA FOR WIDE APERTURES. 233 

objects are fit for the delineation of an image, undisturbed by con- 
fused images from other planes, the ‘defining power’ of an objec- 
tive is always greater with greater aperture for every kind of object, 
inasmuch as under all circumstances the wider aperture admits 
of the utilization of higher amplifications [oculars! Ep.] than can 
be obtained without perceptible loss of sharpness (with the same 
objects) by lower apertures.” 
This is a great admission, and helps our case wonderfully ; let us 
now turn to the opinions of the Editors of the Journal of the Royal 
Microscopical Society :—In vol. I. series II. of the Journal, p. 709, 
in a review of “ The Microscope and its Revelations ” may be found 
the following:—‘ The wider the aperture of an objective the 
greater the technical skill which is required on the part of the 
practical optician; but the notion that as the aperture of an 
objective is increased, its defining power must necessarily, either on 
theoretical or practical grounds be impaired happily belongs to a 
closed chapter of microscopy.” Prof. Abbe lays stress upon the 
fact that the objects are to be “ fit for the delineation of the image,” 
and this is too often overlooked ; objectives are often blamed on 
account of the unsuitability of the preparation or the clumsiness of 
the operator and his methods of illumination. 
Dr. Goring pointed out years ago that “defining power” 
depended upon the comparative destitution of aberration, and if 
defining power increases (in well corrected objectives) with increase 
of aperture, the wider angles must be the better lenses, and more 
free from aberrations. We need not dilate upon this question, as 
the quotations above will be enough for our readers, especially if we 
add a quotation or two from Dr. Carpenter’s work upon the 
Microscope. “If he (the microscopist) be engaged upon difficult 
Biological investigations, he will do well to make perfect definition 
his sine gué non, and to be content with the largest angle that can 
be obtained without a sacrifice of this.” 
On page 198, when speaking of apertures, he says :—“ If the 
aperture be too small the image will be dark; but if it be too 
large details are brought into view (such as the separateness of the 
particles of the vermilion injection) which it is of no advantage 
to see.” There is another way of looking at this question, and 
we ask :—If the aperture is so small that it conceals the fact, that 
particles of vermilion are contained in the vessels, is it not likely 
that it also hides the structural elements for which the histologist is 
searching ? 
With this question of definition we must consider the power to 
stand deep eyepieces. Now we think it will be universally con- 
ceded that wide apertures will stand deeper eyepieces than small 
angles, and for two reasons: in the first place there is greater 
illumination of the field, and secondly the wider the aperture the 
; 
8 eOaOOEOEOEEE—E—E—E—EEEeEeEeEeEEE * . 
