70 
The Presidenfs Address. 
mining tlie available angle of aperture of an objective,, that 
was shown by Prof. Govi^ of Turin^ to the jury of Class 
XIII of the recent International Exhibition. This con- 
sisted in placing the body of the microscope perpendicular 
to a table covered with any dark^ non-reflecting substance^ 
such as green cloth; and having converted the instrument 
into a telescope^ by placing above the eye-piece a suitable 
combination of two lenses (such as the examining-glass 
of Mr. Ross)^ in observing the greatest distance on either 
side at which a clear image of some distinct object^ as 
a narrow slip of white cardboard or paper, laid on the table, 
can be perceived. Half the interval between these two 
points, divided by the vertical distance of the focal point of 
the objective from the surface of the table, will, by reference 
to a table of natural tangents, give half the required angle 
of aperture. 
MM. Nachet and Son, to whom we are indebted for the 
first practicable arrangement of a binocular instrument, 
exhibited a very creditable collection of microscopes. The 
prismatic arrangement for bisecting the visual pencil in 
their binocular is a great improvement on their former 
plan, and produces perhaps as good results as can be ex- 
pected from any plan involving the sj^mmetrical bisection 
of the pencil; but all such contrivances are manifestly 
inferior to the unsymmetrical plan of Mr. Wenham, in 
which one half of the pencil suffers no disturbance, and 
the other half no refraction, as it enters and emerges from 
the prism perpendicularly. A pencil of rays can hardly be 
expected to pass through a prism without encountering some 
disturbance, which will be made evident by impaired defi- 
nition ; but it is well known that two equally perfect images 
are not essential for the production of a satisfactory bino- 
cular efi*ect ; it seems, therefore, to be a necessary inference 
that a better result will be obtained by Mr. Wenham^s 
arrangement than by any of those in which the pencil is 
symmetrically bisected. There is also the further advantage 
that, by simply withdrawing the prism, which is mounted in 
a small sliding frame, the microscope is at once restored to 
its original uniocular form. 
M.M. Nachet also exhibited some ingenious devices by 
which the pencil transmitted by the objective is prismatically 
divided into three or four parts, and directed into as many 
divergent tubes, to enable a like number of persons to view 
an object simultaneously; but the advantage they would 
derive from seeing an object imperfectly together, in prefer- 
ence to seeing it well in succession^ is not very apparent, 
