The President's Addy^ess. 
139 
observations from our assistant-secretary^ Mr. Williams^ on 
a fungoid disease of the grape vine, as noticed by him some 
years previously. 
In the manipulatory and mechanical department, we have 
from Mr. Wenham a paper " On a method of Illuminating 
Opaque Objects under the highest powers of the Microscope." 
This is a problem, the solution of which has been attempted 
by numerous adepts in manipulation with only very partial 
success, until Mr. Wenham accomplished it by means as in- 
genious as they are simple. Of course, with a one-twelfth 
objective, or any higher power, the use of a Lieberkiihn is 
simply an impossibility, while a plane reflector, as previously 
suggested by Mr. Brooke, reflects the light too feebly to be 
of service in most cases ; and furthermore, there is no very 
ready means of intercepting the transmitted ray when a plane 
reflector is used. Mr. Wenham^ s happy idea of making the 
upper surface of the thin glass covering of the object its own 
reflector, by internal total reflection, not only enables us to 
illuminate with the requisite brilliancy, but also renders 
available objects mounted in balsam, or some other medium, 
which would otherwise often be useless for examination by 
very high powers. The principle involved precludes the use 
of specimens that are mounted dry, for this method of ex- 
amination; to those who are acquainted with optics the 
reason will be at once apparent ; to those who are not the 
explanation would be tedious, and occupy more time than 
could be reasonably devoted to it this evening. 
From Mr. Hislop we have been favoured with the descrip- 
tion of a piece of apparatus for compressing objects while 
being mounted ; and Mr. Farrants furnished a paper " On 
defining the position and measuring the magnitude of 
microscopic objects." This latter being a subject that has 
occupied the attention of many microscopists recently, it was 
thought desirable to appoint a committee to consider and 
report upon the best form of a universal finder, so that an 
object being noted by one operator, might readily be found 
by others, although by a different instrument and apparatus. 
The report of the committee will be found in the ^ Trans- 
actions,^ and fully carries out the conditions imposed. 
The consideration of this question involved another point 
of scarcely less importance, or rather gave rise to its 
consideration; the desirability of inducing the various in- 
strument-makers to adopt one uniform size and mode of 
attachment for objectives, so that they may be used upon 
various instruments indiflPerently without the aid of adapters, 
these being found to interfere very materially with the 
