The Presidents Address. 
71 
The only conceivable practical utility of sucli an instrument 
is to enable several persons to observe simultaneously rapid 
chemicalj physical^ or vital transformations. 
M. Hofmann,, of Paris, exhibited a polari-microscope_, an 
ingeniously designed and very convenient instrument for the 
examination of small crystals and crystalline plates^ under 
the influence of polarized light. The object to be examined 
is placed in the middle of the instrument, at the common 
focus of two triple combinations, so constructed as to collect 
the pencils from a large field of view. A polarizer is placed 
beneath the lower triplet, and an eye-piece and analyser 
above the upper one. The visual angle is so large that the 
two axes of biaxial crystals may frequently be viewed simultane- 
ously, even when separated by a considerable angular interval. 
This appears to be the most complete and effective apparatus 
that has been constructed for this class of observations. 
M. Nobert, of Berlin^ exhibited a microscope of his own 
design, and his well-known test lines. The microscope was 
not conspicuous for the convenience of its arrangements ; 
it is tall and vertical, and has a micrometer stage-movement, 
consisting of a micrometer-screw with a large graduated 
head attached to an adjacent ^<2?e(i pillar, and connected with 
the stage by a Hookas joint, in order to admit an adjustment 
of the stage for focusing. The vertical position of a micro- 
scope is ahvays undesirable, as the necessarily flexed position 
of the head incommodes the circulation of the blood, 
and tends, in conjunction with the active exercise of 
vision, to produce congestion in the eye; moreover, vision 
is liable to be rendered indistinct by the gravitation of anj?- 
humours, floating on the surface of the eye, to the then lowest 
point, the centre of the cornea. 
Having been for some time past occupied in endeavouring 
to determine the best method of obtaining very high magni- 
fying power, I am induced to lay before you the results at 
which I have at present arrived, in the hope that they may 
not be altogether devoid of interest, and, moreover, that 
some of my hearers, who have had much more experience 
than myself in the use of the microscope, may be induced to 
co-operate in determining this important practical question. 
Three different modes of augmenting magnifying power 
have been under consideration : 
I. By increased power of the objective ; 
II. By increased length of tube; 
III. By increased power of the eye-piece.'^ 
* To these migiit be added a fourth method, that of doubling the objec- 
tive, and magnifying b^ the second the image obtained from the first ; tliis 
