The President's Address. 
57 
sufficiently large to admit the full pencil from the object- 
glass^ through the back of which the rays from a lamp 
(passing through a hole in the side of the body) were reflected 
downwards. The object was strongly illuminated^ but there 
was so much glare from the internal fittings, and from reflec- 
tion from the back of the object-glass lenses, that the experi- 
ment was abandoned, and an unfavorable opinion given of its 
practicability. It is now demonstrated that the light was too 
intense, and the most useful or central portion of the rays 
were wanting. The simple disc of thin glass and its partial 
reflection meets these objections. If such a disc is used with 
a little care it is found to be quite as accurate as the other 
plan, and the natural surface of the glass is more reflective 
than any artificial one. It has, however, the disadvantage of 
extreme fragility. By making the object-glass its own con- 
denser, and examining diatoms as opaque objects under high 
powers, we can now hope to solve the much vexed question 
as to the true nature of their markings. Mr. Browning has 
employed the apparatus in a form much more nearly resem- 
bling that of the original inventor, only substituting a small 
glass-reflecting prism for the metallic reflector. Some ad- 
vantages are gained by the adoption of this arrangement, 
which, I believe, Mr. Browning will describe in a short paper 
at our next meeting. 
Mr. Hewitt exhibited, at a recent meeting of the Society, 
a plan in which one tube of a binocular instrument was 
surmounted by a small flat mirror which sent a pencil of light 
vertically down one tube, and then, by means of the prism 
employed in the binocular arrangement, down through the 
other. 
The objections to this plan are, first, that the prism cuts 
off a large portion of the aperture ; secondly, that it cannot 
be used when binocular vision is desired ; and, thirdly, that 
it is not adapted to very high powers, as microscopists are 
agreed that when a prism is used with great powers it must 
be placed close to the optical combination, and not at the 
distance from them in which it occurs in the ordinary binocu- 
lar arrangements. This plan may, however, be liked for its 
simplicity by many who do not desire extreme magnification, 
and who operate with highly reflective objects. 
These new modes of illumination bid fair to correct many 
errors of interpretation resulting from an exclusive use of 
transparent illumination, and we hope that in the hands of 
the members of this Society they will reveal many peculiari- 
ties of structure as yet unknown. 
Since the arrival in this country of the condenser sent by 
