The President’s Address. 
31 
whole of the aperture of the object-glass may be obtained in 
each eye with the highest powers. The first plan for this 
purpose was brought before us by Messrs. Powell and 
Lealand, the second by Mr. Wenham: neither of them is 
intended to produce the peculiar stereoscopic effect resulting 
from the combination of two views of the same subject taken 
at different angles, but they have been devised to secure the 
physical convenience of using both eyes at the same time. 
To obtain an image by the whole aperture of the object- 
glass in each tube, Messrs. Powell and Lealand interposed an 
inclined disc of glass with parallel sides, so that one set of 
rays from the object are transmitted direct through it, while 
another portion of the light is reflected by the glass surface, 
and again reflected into the second tube ; or, in other words, 
one portion of the light proceeds through the glass disc up one 
tube, while another portion reflected from it, suffers a second re- 
flection from a rectangular prism, and is directed up the other 
tube. Considerable success is obtained by this method. Messrs. 
Powell and Lealand are able to display by it both sets of lines 
on the Pleurosigma rhomboides with only an infinitesimal 
loss of definition, yet the difference in brightness between 
the two images — one formed by transmitted, and the other 
by reflected light — is considerable. Mr. Wenham, noticing 
this great difference between the amount of light sent to 
each eye, called attention to the fact that at an angle 
of 45°, only 53’66 out of 1000 incident rays, or about 
th part, could be reflected from a glass surface ; and to 
obtain a more equal illumination, he devised the highly 
ingenious combination of prisms described and figured in 
the f Proceedings’ of the Society, as reported in the f Quar- 
terly Journal of Microscopic Science’ for July, 1866. Mr. 
Wenham’s remarkable manipulative skill enabled him to 
realise to a very important extent all the results he anticipated, 
but our leading professional artists consider his plan difficult 
of execution, and have not yet made the new combination of 
prisms for sale. 
This is to be regretted, because there can be no doubt of 
the merit of his invention, and it seems to offer the best mode 
of avoiding the bad effects of a too exclusive use of a single 
eye, which, as is well known, tends to derange its power of 
focussing consentaneously with the other eye. It is also 
probable that in the prolonged examination of objects, better 
vision would be obtained by two eyes than one, as the organ 
of vision would be less fatigued. 
Mr. Wenham in his paper embodied several plans in which 
the reflection was obtained from two contact surfaces, by 
