The President's Address . By E. M. Nelson 4 
125 
well remember the form of the student’s instrument when I first took 
up the Microscope. The student’s Microscope of that day consisted of 
a very short tube Hartnack, with a very bad direct-acting screw fine- 
adjustment, a push-tube coarse-adjustment, a stage with a small hole 
in it, no substage or condenser of any kind, a concave mirror, a 
bull’s-eye fixed to the limb for superstage illumination, a separating 
objective composed of three French buttons, and two eye-pieces, 
packed in a box. 
Messrs. Swift and Son were the first in this country to alter this 
state of things. They brought out a better made Microscope, mounted 
on a bent claw foot, fitted with cheap objectives of a far superior quality, 
both with respect to power and aperture. Being much struck with the 
improvements they had effected, I instructed them to make for me a 
similar instrument, fitted with a rack-and-pinion coarse adjustment 
and a condenser. This, I believe, was the first student’s Microscope, 
properly so called, that had a rackwork coarse-adjustment, and a very 
efficient instrument it proved to be. It was with this instrument 
that the beaded structure of tubercle bacilli was first demonstrated, 
on May 23rd, 1882. 
Since that time, however, improvements in the student’s Micro- 
scope have gone on apace, culminating in the fine models you have had 
exhibited before you during the past session. Almost all student’s 
Microscopes now have rackwork coarse adjustments, and many are 
fitted with substage condensers of some form or other. Messrs. 
Watson have fitted to their student’s Microscope a lever fine-adjust- 
ment on a thoroughly sound principle, thereby raising it to the 
position of a Microscope that can, for the first time with truth be 
called “ good enough for histological purposes.” 
Dispersion. 
It will be in the recollection of the Fellows that the subject of 
my previous Address, selected for your kind consideration, was far too 
extensive to be dealt with on that occasion ; therefore only a review 
of what was termed ‘‘the middle portion” was placed before you. 
To-night I propose to introduce briefly and inadequately the “ first 
portion,” viz. Dispersion. This, in itself a very large subject, has 
occupied the attention of many eminent mathematicians, physicists, 
and opticians ; still the last word has not been spoken ; for it is ad- 
mitted by the ablest writers that as yet all the phenomena have not 
been fully explained, neither is the matter fully understood. 
It is not my intention to push the inquiry any further into the 
unknown, even if I were qualified to do so, which I am not ; but I 
shall endeavour to put some of the received facts before you this 
evening in what probably is a new and original dress, which I hope 
may prove helpful to those who are anxious to know something of 
dispersion, and yet have neither the time nor the opportunity to 
study the larger treatises on the subject. 
