PROFESSOR MARTIN DUNCAN’S ADDRESS. 337 

find the best media with high refractive indices, he has quite lately 
brought before the Society the utility of an agueous fluid capable 
of being given the high refractive index of 1.68, viz. a solution of 
biniodide of mercury and iodide of potassium in distilled water. 
This more manageable and highly antiseptic medium appears likely 
to turn out to be of great use in the observation of many objects, 
as its strength can be diluted till the index of water is obtained. 
This is of advantage with such objects as muscular fibre, which are 
themselves of high refractive power, so that fluids of ow refractive 
power must be made use of to obtain the required difference for 
more perfect visibility. The same communication also contains 
what was much wanted, detailed practical directions for mount- 
ing. 
Any one who has seen the diatoms and scales mounted in phos- 
phorus by Mr. Stephenson’s method, and exhibited at our meetings 
during the past and present sessions, cannot fail to have been 
struck by the great increase in their visibility as compared with 
those mounted in balsam, or to have recognized the fact, that the 
theoretical consideration by which their visibility was pronounced 
to be much increased, was not unfounded. 
In addition to the increase in visibility, there is also the fact 
that by means of such mounting fluids, the capacity of stereoscopic 
binoculars with the higher powers is considerably enhanced. True 
stereoscopic effect, as we have seen, requires a depth of vision not 
less than the thickness of the object under observation—a depth 
which, as already shown, increases in direct proportion with the 
increase in the refractive index (7) of the mounting fluid. If one 
object is in air when # = 1.0, whilst another is in a solution of 
phosphorus, where 2 = 2.1, the depth of vision will be more 
than doubled. Objects, therefore, that by reason of their thickness 
could only afford an unsatisfactory stereoscopic effect in air may be 
seen in full relief when mounted in phosphorus. 
Here, again, the deductions of theory were remarkably verified 
by the recent exhibition of Surivel/a gemma, under the binocular, 
with a 3,-inch objective. 
Relative Value of Objectives with Large and Small Apertures. 
(“All-round Vision.’) 
I now come to a much-vexed question, that of the relative value, 
practically, of objectives of large and small apertures, in regard to 
which a great variety of opinions have been promulgated. 
The oldest of these views was that which made the preference 
between the two kinds of objectives, depend upon whether they 
were to be used for the “ordinary purposes of the biologist,” or 
for the examination of diatoms or other lined objects. The objec- 
tion to this view is, that it assumes the only function of a large 

