86 BROOKES, ON MICROSCOPES AT THE EXHIBITION. 
phragm inwhicharemarginal apertures, capable of adjustmenteither 
to an interval of 90° with each other, when the arrangement of 
the dots to be developed is quadrangular, as in WV. rhomboides, or 
P. hippocampus ; or to one of 60° and 120°, when they are arranged 
in equilateral triangles, as in P. angulatum, &c. An ingenious 
plan of illuminating by reflected light minute objects, mounted in 
Canada balsam, has been devised by Mr. Wenham; this consists 
of a small truncated glass paraboloid, which is temporarily 
attached to the under side of the slide containing the object, by a 
little gum, oil, or fluid Canada balsam. The rays internally reflected 
from the convex surface of the paraboloid, and impinging very ob- 
liquely on the under surface of the slide, are transmitted in con- 
sequence of the fluid uniting medium, and are then internally 
reflected from the upper surface of the covering glass on to the 
object. Very minute variations of surface contour may by these 
means be rendered evident. 
Considerable improvements in the brass-work have been 
recently effected; in the first-class instruments of Mr. Ross, and 
of Messrs. Powell and Lealand, the rotating stages are most con- 
veniently arranged. It must be borne in mind that the solidity and 
weight of material necessary to entirely obviate tremor, when high 
powers are used, is incompatible with portability. On this ac- 
count more portable forms of stand have been constructed 
by most of the principal makers ; a stand made by Mr. Ladd has 
been considered to combine lightness and portability with as much 
rigidity as is compatible with the weight of material employed. 
Most praiseworthy endeavours have been made by many ex- 
hibitors to produce an efficient instrument at a price compatible 
with the means of students and others, to whom a first-class in- 
strument is unattainable. 
By the aid of an extensive plant of machinery, Messrs. Smith, 
Beck, and Beck, have succeeded in producing a complete and 
efficient binocular microscope, at the price that is commonly 
charged by the first makers for merely rendering a first-class in- 
strument binocular. They have also constructed a still cheaper 
form of instrument, combining great steadiness with fair optical 
efficiency. Mr. Highley and Mr. Pillischer have also greatly dis- 
tinguished themselves in this department. Very cheap forms of 
compound microscope are exhibited by Mr. Field, who obtained the 
Society of Arts’ prize some years since, but who does not appear 
to have in any respect improved his model; and others by Mr. 
Parkes, the cheapest of all, but at the same time it must be 
added, the least efficient optically ; whether the quality is as good 
as can be procured at the price is a question which none but the 
manufacturer can determine. 
There is a very creditable display of preparations, both British 
and foreign; but it is to be regretted that one, who has for many 
years been considered the first British preparer, has contributed 
nothing to this Exhibition. The German objects prepared by 
imbibition and transparent injection, imported and exhibited by 
