338 THE NORTHERN MICROSCOPIST. 

aperture to be its resolving power, a much too restricted notion, 
and one which deprives the working biologist of a most essential 
aid to his observations upon structure. 
A more modern view errs in the opposite direction, and insists 
upon the universal superiority of large apertures, so that work done 
with small apertures will “ have to be done over again.” 
There is again a third view, still more recently put forward, 
which goes much further than the preceding, and according to 
which it is impossible that wide apertures can give correct images. 
First on account of the unnatural “all-round vision” which it is 
contended is obtained with them, and secondly by reason of their 
supposed inherent defect in defining power, in consequence of the 
dissimilar images presented by the different parts of the enlarged 
area of the objective, with a confused image as the general 
resultant. 
The want of exactness in the first two suggestions will suffi- 
ciently appear, when we have formulated the grounds upon which 
large apertures are shown to be indispensable for all observations 
upon minute structure for which high powers are necessary ; but it 
will be desirable first to point out the erroneous interpretations 
upon which the third view (as to all-round vision and dissimilar 
images) has been founded, and for this purpose it will be necessary 
to refer to the paper by Dr. Royston Pigott, F.R.S., in which the 
subject is dealt with.* 
After reminding his readers that he had shown that spider-lines, 
miniatured down to the fourteenth part of the hundred-thousandth 
of an inch, were distinctly visible to ordinary good eye-sight under 
proper microscopical manipulation (an experiment which, I may 
remark in passing, has not a satisfactory foundation), Dr. Pigott 
says :—‘‘ Under these circumstances it was interesting to know 
whether real objects could be detected by the Microscope in the 
surprising degree of attenuation represented by the millionth.” 
Minute particles of mercury were obtained by smashing some with 
a watch-spring, and they were mounted in petroleum under a thin 
cover. A vertical illuminator was used to converge rays down- 
wards, through the objective, upon the preparation. Ina darkened 
room minute disks became visible, and upon some of them clusters 
of minute black points were seen with a power of 1000 diameters. 
Comparing them with a micrometer spider-line j,4,, inch 
diameter, some of the points were found to be decidedly smaller. 
Under 1000 diameters the particle was magnified one hundred 
times in the micrometric focus, and then appeared less than the 
top Of z5d05 inch, or less than the millionth of an inch, and the 
writer draws the conclusion that “real objects of unsuspected 
Pca haa a bin A Bi bn Se oe me a 
* Proc. Roy. Soc., xxxi (1881) pp. 260-78, 
