103 
On a Binocular Microscope for High Powers. 
By F. H. Wenham. 
("Read May 9th, 1866.) 
The common binocular microscope performs satisfactorily 
up to the ^, but for powers above this a special arrangement 
is needed for the prism_, which must be set close behind the 
posterior lens of the -^th, tV'^^^ upwards, in order to obtain 
an entire field of view in each eye. This it is found to 
accomplish perfectly when in that position; but still for very 
delicate test objects requiring the utmost extent of aperture 
for their definition, it will not resolve them as clearly as with 
the single body, from the fact that the aperture is divided 
and half only effective in each eye. It has therefore long been 
thought desirable to obtain the whole aperture in each tube. 
This has recently been effected by Messrs. Powell and Lealand 
by means of an inclined disc of glass with parallel sides ; the 
partial reflection from the under surface is again reflected 
into the second eye by means of a rectangular prism. Assum- 
ing this surface to be placed at an angle of 45°, the amount of 
reflected light will be only 53-66 out of 1000 of the incident 
rays, or nearly tV^^ part. To collect half the light the 
reflector would have to be set at 82^°, but this would cause 
the glass plate to extend to such a length as to render the 
adaptation nearly impracticable, but even with the above- 
named enormous difference in the relative quantity of light, 
the arrangement as turned out by the hands of these clever 
mechanicians has surprised us with the fact that a good effect 
may be obtained by such means, and having thus started 
the principle it remains to be seen what improvements 
can be made with the view to increasing the quantity of 
reflected light, and if possible obtaining a more equal result 
in each eye. 
By slightly modifying the existing arrangement of Messrs. 
Powell and Lealand, light otherwise lost may be utilised. 
In order to prevent the image from the second surface of the 
reflecting disc from appearing at the eye, and overlapping 
and confusing the first, Messrs. Powell and Lealand make it 
of considerable thickness. The secondary image is thus so 
far separated as to be thrown beyond the reach of the 
rectangular prism, and practically this light is totally lost. 
As the disc is made thinner, so do the images approximate and 
the distance between them diminish. Therefore if the glass is 
made as thin as practicable, and a very slight angle given to 
