Wheatstone on the Binocular Microscope, Sfc. 101 
minute description of the instrument, which he informs us he 
constructed, and presented to the Dauphin. The following is 
an extract from the Preface : — 
" Some years ago I resolved to effect what I had long before premedi- 
tated, to make a microscope to see the smallest objects with the two eyes 
conjointly ; and this project has succeeded even beyond my expectation, 
with advantages above the single instrument so extraordinary, and so 
surprising, that every intelligent person to whom I have shown the effect 
has assured me that inquiring philosophers will be highly pleased with 
the communication. For this reason I have determined to make it the 
principal subject of the present work." 
And the second part, which contains a description of the in- 
strument, is thus headed : — 
" Section the first, in which is taught the method of constructing a 
newly-invented microscope to see the smallest objects very agreeably and 
conveniently, represented entire to the two eyes conjointly, with a magni- 
tude and distinctness which surpasses everything which has been hitherto 
seen in this kind of instrument." 
In the Pere d'Orleans' binocular microscope, two object- 
glasses have their lateral portions cut away so as to allow of 
close juxta-position, and these nearly semi-lenses are so 
arranged, that their axes correspond with the two optic axes 
passing through the tubes containing the eye-pieces. The 
author's aim in its construction was solely the reinforcement 
of the impression by presenting an image to each eye, for he 
assumes, according to the then prevalent error, that vision by 
the two organs conjointly is naturally and necessarily unique, 
from the perfect conformity of all the homonymous parts of 
the two images of the object on the two retinae. The real ad- 
vantage of such an instrument entirely escaped his attention ; 
viz., that of presenting to the two eyes the two dissimilar 
microscopic images of an object, under precisely the same cir- 
cumstances as the two unlike images of any usual object is 
presented to them when no instrument is employed, by which 
simultaneous presentment the same accurate judgment as to 
its real solid form, and the relative distances of all its points, 
can be as readily determined in the former case as in the latter. 
In the construction of a binocular microscope there is one 
thing especially to be attended to — viz., that the images be both 
direct, for in this case only a true stereoscopic representation 
will be obtained. If the images, on the contrary, be inverted, 
a pseudoscopic effect would be produced which will give a very 
erroneous idea of the real form. The reason of these effects is 
fully explained in Sections 5, 10, 22, 23, of my Memoirs. The 
reversal of the images by reflection from mirrors or reflecting 
prisms, will produce the same result as to the stereoscopic and 
pseudoscopic appearances as their inversion by lenses. The 
binocular microscope constructed by the Pere d'Orleans was 
