BROOKES, ON MICROSCOPES AT THE EXHIBITION. 85 
glasses for that especial purpose ; the labour of construction being 
immensely augmented by the difficulty of duly balancing the aber- 
rations of the more oblique pencils. So much is this the case, 
that in the best constructed objectives of the largest angles, the 
visual effect is sensibly impaired when the rays are transmittod 
through any other thickness of covering-glass than that for which 
they have been specially corrected. 
The introduction of the binocular arrangement of Mr. Wenham 
has created quite a new era in the history of the microscope. 
This ingenious contrivance consists in intercepting one half of 
the pencil emerging from the object-glass by a prism placed im- 
mediately above it, the transverse section of which is a trapezium, 
of such form that the transmitted half-pencil is made to form the 
usual visual angle with the undisturbed half; the surfaces of in- 
cidence and emergence are both perpendicular to the respective 
directions of the rays, which suffer two internal reflections in pass- 
ing through the prism. 
A binocular arrangement was some years since constructed by 
M. Nachet, of Paris, which has been considerably improved in the in- 
struments recently exhibited by him ; and another was exhibited by 
Mr. Dancer, of Manchester, which closely resembles a plan pre- 
viously designed and abandoned by Mr. Wenham. In both of 
these the pencil is bisected by the double prism, and the two halves 
diverge equally in opposite directions. As, however, the pencils of 
rays can hardly be expected to pass through a prism without some 
sensible disturbance, and as it well known that the superposition of 
two equally perfect images is not essential for the production of 
a satisfactory binocular effect, it seems most probable that a better 
result will be obtained by the construction of Mr. Wenham than 
by either of the others; and it has this further advantage, that by 
simply withdrawing the prism, which is imtroduced in a small 
sliding frame, the microscope is at once reduced to its original mo- 
nocular form. 
Several new modifications of illuminating apparatus have been 
introduced in this country since 1851; the principal of these are 
a condenser of very large angular aperture, by Messrs. Powell and 
Lealand, in which every requisite modification of the illuminating 
pencil may be produced by two revolving discs, one containing 
apertures of various sizes, and the other various diaphragms for 
excluding the central portion, or for admitting only angular por- 
tions, of the pencil of light. These dises are placed immediately 
below the posterior lens of the illuminator. This method of modi- 
fying the illuminating pencil was first applied in Gillett’s con- 
denser, as constfucted by Mr. Ross, by whom this ‘latter 
apparatus has recently been modified for the purpose of affording 
a more efficient illumination for medium powers. A hemispheri- 
cal condenser has been produced by the Rev. J. B. Reade, which 
answers remarkably well for the purpose for which it was devised, 
namely, the development of the markings of diatoms. The plane 
surface of the hemisphere is placed upwards, and is covered by a dia- 
