106 Dr. Carpenter, on Stereoscopic Binoculars. 
after suffering reflexion within it, are transmitted upwards 
into the right - hand body, r', passing out of the prism 
perpendicularly to the plane of emersion, which has such 
an inclination that the right-hand or secondary body 
(r, Fig. 2) may diverge from the left or principal body at a 
suitable angle. On the other hand, the right half (r) of the 
cone of rays passes upwards, without essential interruption, 
through the two parallel surfaces of the prism a, into the 
left-hand body ( l '), and is thus crossed by the others in the 
interior of the prism. But if the prism A be pushed over 
towards the right (by pressing the button «, Fig. 2), so as 
to leave the left half of the objective uncovered, as 
shown in position 2, Fig. 1, that half (/) of the cone of rays 
will go on without any interruption into the left - hand body 
(/'), whilst the right half (r) will be reflected by the oblique 
face of the prism into the horizontal direction, will emerge 
at its vertical face, and, being received by the second prism, 
B, will be directed by it into the right-hand. body (r'). The 
adjustment for the distance between the axes of the eyes is 
made by turning the milled-head b (Fig. 2 ), which, by 
means of a screw-movement, acts upon a movable chariot 
that carries the prism b and the secondary body r, the base 
of which is implanted upon it. Now, in the first\ position, 
the two halves of the cone of rays being made to cross into 
the opposite bodies, true Stereoscopic relief is given to the 
image formed by their recombination, just as in the ordinary 
arrangement. But when, in the second position, each half 
of the cone passes into the body of its own side, so that the 
reversal of the images produced by the Microscope itself is no 
longer corrected by the crossing of the two pencils separated 
by the prism a, a Pseudoscopic effect, or “ conversion of 
relief,” is produced, the projections of the surface of the 
object being represented as hollows, and its concavities 
turned into convexities. The suddenness with which this 
conversion is brought about, without any alteration in the 
position either of the object or of the observer, is a pheno- 
menon which no intelligent person can witness without 
interest, whilst it has a very special value for those who 
study the Physiology and Psychology of Binocular vision* 
* The result of the numerous applications which the Author has made 
of this instrument to a great variety of Microscopic objects, has led to a 
confirmation of the principle of Pseudoscopic vision which he has stated 
elsewhere — viz. that the readiness with which the conversion is effected 
depends on the readiness of the mind to apprehend the converted form. Where, 
as in the case of the saucer-like discs of the Arachnoidiscus , the real and 
the converted forms are equally familiar, the “conversion” either of the 
convex exterior or of the concave interior into the semblance of the other 
