108 Du. Carpenter, on Stereoscopic Binoculars. 
to his Dissecting Microscope,* he has found its utility to 
be practically limited by the narrowness of its field of 
view, by its deficiency of light and of magnifying power, 
and by the inconvenience of the manner in which the eyes 
have to be applied to it. An arrangement greatly superior 
in all these particulars having been recently worked out by 
MM. Nachet, the Author has combined the Optical part of 
their Dissecting Microscope with Mr. R. Beck’s Stand, and 
finds every reason to be satisfied with the result ; the solidity 
of the Stand giving great firmness, whilst the size of the stage- 
plate affords ample room for the hands to rest upon it. The 
Objective in Nachet’s arrangement is an Achromatic combi- 
nation of three pairs, having a clear aperture of nearly 3-4ths 
of an inch, and a power about equal to that of a single lens 
of one-inch focus ; and immediately over this is a pair of 
prisms, each resembling a, Fig. 1 2 , having their inclined 
surfaces opposed to each other, so as to divide the pencil of 
rays passing upwards from the Objective into two halves. 
These are reflected horizontally, the one to the right and the 
other to the left ; each to be received by a lateral prism cor- 
responding to b, and to be reflected upwards to its own Eye, 
at such a slight divergence from the perpendiculars as to give 
a natural convergence to the axes when the eyes are applied 
to the Eye-tubes, superposed on the lateral prisms — the dis- 
tance between these and the central prisms being made 
capable of variation, as in the Compound Binocular of the 
same makers. The magnifying power of this instrument may 
be augmented to 35 or 40 diameters, by inserting a concave 
lens into each Eye-piece, which converts the combination into 
the likeness (as originally suggested by Professor Briicke, of 
Vienna) of a Galilean Telescope (or Opera-glass) ; and this ar- 
rangement has the additional advantage of increasing the dis- 
tance between the object and the object-glass, so as to give 
more room for the use of dissecting instruments. 
To all who are engaged in investigations requiring very 
minute and delicate Dissection, the author can most strongly 
recommend MM. Nachet’s instrument. No one who has not 
had experience of it can estimate the immense advantage 
given by the Stereoscopic view, not merely in appreciating 
the solid form of the object under dissection, but also in 
precisely estimating the relation of the instrument to it in 
can only be obtained by making those bodies diverge at an angle so wide as 
to produce great discomfort in the use of the instrument, from the neces- 
sity of maintaining an unusual degree of convergence between the axes o. 
the eyes. 
* ‘ Transactions of the Microscopical Society,’ N. S., vol. xii, p. 3. 
