






206 THE NORTHERN MICROSCOPIST. 

2. The utilization of a given aperture depends in principle on the amplification 
of the ultimate image which is projected by the entire Microscope to the 
observer’s eye. Now one and the same amplification may be obtained in very 
different ways since it is the resultant of three distinct elements, (a) focal length 
of the objective, (4) focal length of the ocular, and (c) length of the tube. Any 
definite number of diameters (say 1000) can be obtained with a low power 
objective (say a 1-inch) as well, from a mere dioptrical point of view, as with a 
higher power (say %%-inch), by applying a sufficiently deep eye-piece and a 
sufficient length of the tube. It is, however, well known that there is a great 
difference in the optical qualities of images which are produced under these 
different conditions. Forcing a high amplification from a low-power objective 
is always connected with a considerable loss of sharpness of definition of the 
image, owing to the magnification of the residuary aberrations, which are 
inherent even in the most finished constructions. It is, therefore, a well- 
established practical rule that a certain amount of amplification requires a certain 
power of the objective—higher amplification a higher power (shorter focal 
length)—in order to obtain the image under those favourable conditions which 
are necessary for their full effectiveness. This considered, the inference of the 
foregoing paragraph may be expressed in these terms :— 
Wide apertures with objectives of short focal length; low and moderate 
apertures with objectives of low and moderate power. 
As a detailed discussion of this subject will be found in the second part of 
this paper, it will be sufficient here to point out some notable facts of experience 
by way of example only. 
With objectives of say 1 inch, and ¥% inch, focal length, the lower and 
medium eye-pieces in use will yield 40-80 and 80-160 diameters only. In order 
to obtain 150 and 300 respectively, very deep oculars (or an extra length of the 
tube) would be required. So far now as such objectives are intended for the 
lower powers mentioned above, an aperture of about 0.15 (18°) in the case of 
the 1-inch, and of 0.3 (35°) in the case of the ¥4-inch, are at all events more 
than sufficient for showing every detail which can possibly be recognized by the 
eye under these amplifications, and therefore wider apertures are useless. In 
point of fact, no observer will see anything more or anything better with 
similar objectives of say 0.40 (48°) and 0.75 (96°) respectively, than with the 
narrower angles indicated above, as long as the low and medium oculars are in 
question only. These latter apertures would require for their full utilization, 2.2. 
for convenient observation of the minuter details which are within their reach, 
amplifications of much more than 1 50 and 300 diameters. With well-made 
objectives of those apertures, such figures may be realized indeed, and details 
may be shown by means of deeper eye-pieces, which remain quite invisible with 
the lower angled systems; but no microscopist can deny the inferior quality of 
the images obtained in this way if compared to those of equal amplification, 
which are obtained with these same apertures when the objectives have double 
the power and the oculars the half only. Structures of so simple a composition 
as diatom striae may perhaps be tolerably displayed under such forced amplifica- 
tions of low-power objectives, but with objects of somewhat irregular and com- 
plicated structure the deterioration of the image attendant upon a considerable 
enlargement of the residuary spherical and chromatic aberrations by deep eye- 
pieces, becomes at once obvious even with the most finished objectives. In 
point of fact, no experienced histologist will ever use in ordinary work even an 
ocular amplification of the amount necessary for obtaining 100 diameters from a 
I-inch objective or 200 from a ¥-inch. He would be unwise if he troubled him- 
self with inferior images whilst good images of the amplifications required 
could be obtained with equal, or even ‘greater, convenience with objectives of 
the same apertures but half the focal length. 

