Wen HAM on Binocular Vision, 
11 
draw tubes and the usual eye-pieces, dd. The distance between 
them should be rather less than the average distance asunder 
of the eyes, and in cases where 
these are very wide apart we can 
pull out the draw tubes, which will 
increase the distance between the 
eye-pieces. 
With this apparatus I obtain the 
whole of the field of view in each eye, 
which circumstance I was not pre- 
pared to expect, as this must in 
some measure depend upon the cor- 
rection of the oblique pencils of the 
object-glass, for we cannot expect 
to look obliquely through the ob- 
jective of a compound achromatic 
microscope in the same way as in 
the single lens arrangement, fig. 1, 
but can only avail ourselves of such 
oblique pencils of rays as are cor- 
rected for passing through the axis 
of the microscope. The arranfje- 
ment represented by fig. 10 certainly 
gives a larger and better field than 
any other that I have yet tried ; and 
on examining a globule of mercury 
I could not discover any aberration or inward or outward 
coma when viewed by the eyes, either separately or together. 
I should here mention that the same illusion is occasionally 
produced in the appearance of some objects with the instru- 
ment last described, as mentioned by Mr. Riddell, the visicm 
being to some eyes pseudoscopic, or projections appearing as 
depressions, et vice versa. Probably habit would enable us to 
judge of their true form without our being under the necessity 
of resorting to a special expedient for the removal of the de- 
ception. 
I have not yet tried a binocular polariscope applied to this 
instrument, but I have reason to expect some curious effects 
from it. 
I have thus far announced the progress of my experiments 
towards the attainment of complete binocular vision with the 
microscope, and I cannot too strongly insist on the importance 
of striving to arrive at a perfect result, particularly with the 
highest powers, for I feel convinced that it will be the means 
of settling many disputed points of structure. Whether it will 
require objectives of a peculiar construction I am not at present 
able to determine, but I may observe that the high power 
