94 BROOKES, ON MICROSCOPES IN THE EXHIBITION, 
magnifying power is thrown principally on the anterior, and 
the correction of aberration on the middle and posterior com- 
binations. 
J. G. Hormany, Paris M. (France, 1440), exhibits a polari- 
microscope, an ingeniously designed and very convenient mstru- 
ment 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 pen- 
cils 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 bi-axial 
crystals may frequently be viewed simultaneously, even when 
separated by a considerable angular interval. This appears to be 
the most complete and effective apparatus that has been con- 
structed for this class of physical investigations. 
Nacuet & Son, Paris, M. (France, 1416), exhibit a good col- 
lection of instruments, of which their binocular microscopes are 
the most conspicuous. M. Nachet has undoubtedly the credit of 
having been the first to achieve the successful construction of a 
binocular microscope. The prismatic arrangement for bisecting 
the visual pencil in the instruments recently exhibited is far 
superior to that previously adopted by the same firm, and yields 
perhaps as good a result as can be expected from any symmetrical 
plan of construction ; the reasons for preferring the wasymmetrical 
plan of Mr. Wenham have already been assigned. This firm 
also exhibits some ingenious devices by which the pencil trans- 
mitted by the objective is prismatically divided mto three and 
four parts, and directed through as many divergent tubes, to 
enable a like number of persons to view an object simultaneously ; 
but the advantages which such persons would derive from seeing 
an object imperfectly together, in preference to seeing it well 
in succession, is not very apparent. 
F. A. Nosrrt, M., Berlin (Prussia, 1410), exhibits a micro- 
scope of his own design, and his well-known test lines, for which 
a prize medal was awarded in the Exhibition of 1851, and a 
description of which will be found at page 268 of that Jury 
Report. The microscope is 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 fired pillar, and connected 
with the stage by a Hook’s joint, in order to admit an adjustment 
of the stage for focusing. ‘The vertical position of a microscope 
is always undesirable, where it can be avoided, 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; moreover, vision is liable to be rendered 
indistinct by the gravitation of any humours floating on the 
surface of the eye to the then lowest point, the centre of the 
cornea, 
