12 
Wenham on Binocular Vision. 
object-glasses, as now constructed, are best suited for viewins: 
very thin objects. We obtain far more pleasant vision of 
bulk and depth with a smaller aperture. I have no doubt 
that the defects of the larger aperture arise from the confused 
medley of stereoscopic images blended together in one eye, 
and which confusion must increase with increase of aperture ; 
but if, on the other hand, we can divide these images between 
both eyes, then I admit that the aperture cannot be too great, 
as the largest portion of microscopic objects, from the way in 
which they are mounted, would be all the better shown under 
an exaggerated perspective, if I may so express it. 
The binocular microscope has already explained to me 
some of the false appearances arising from oblique illumina- 
tion. I refer particularly to what is known as the diffractinc/ 
spectrum ; for example, if we illuminate the Podura by very 
ohlique light, we see a kind of overlyins: shadow, upon 
which the markings of the scale are also visible. As 1 can- 
not reconcile this appearance to the known laws of the dif- 
fraction of light, I think that it is miscalled, and appre- 
hend that the phenomenon merely arises from the oblique light 
illuminating one of the perspective images partly as an 
opaque, and the other as a transparent object, and that they are, 
consequently, so far separated as to give the appearance of a 
double image. 
In illuminating objects under the binocular microscope with 
the ordinary concave mirror some management is required in 
order to get both images equally intense, for we can readily 
get one brilliantly illuminated, while the other is in compara- 
tive darkness, appearing on a black ground almost as an 
opaque object, exactly resembling in their combined effect on 
the eyes what is known as the diffracting spectrum. It also 
occasionally happens that the angle of light from the mirror 
is not sufficient to illuminate both images at the same time. 
These appearances lead me to conjecture that the instrument 
will require a particular kind of illumination, but I am hardly 
yet in a position to express a decided opinion on the subject, 
but will investigate the matter shortly. 
This is the sum and substance of my present experience 
with respect to binocular vision applied to the microscope, 
and I do not think that mere enthusiasm has led me to over- 
rate the importance of the subject, but hope that what has 
already been done is only the commencement of a new era in 
the advancement of this useful and important instrument. I 
believe that there is yet much to be looked for in the way of 
improvement by the investigation of unexplored optical com- 
binations and principles. 
