884 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
that the order in which the rays proceeding from the sides of the object 
or image viewed reach the eyes, as to right and left, is reversed from 
that which exists in natural vision ; the left eye receiving a prepon- 
derating portion from the right side, and the right from the left side of 
the object. 
It is to be noted, however, that tbe eye-piece with rectangular prisms, 
arranged after the first method of Prof. Eiddell, does not uniformly 
produce conversion of relief, or that inversion of perspective which 
obtained in that first experimental arrangement for a binocular Micro- 
scope. Such a binocular eye-piece used in the Microscope upon trans- 
parent objects only occasionally gives the view in depth thus inverted. 
With low powers, and considerable thickness of the transparent object, 
the view is usually psendoscopic. With medium and high powers, it 
is otherwise ; and the effect is much controlled in this respect by the 
direction of the light upon the object. 
When the binocular eye-piece with rectangular prisms is used in 
the telescope to view a landscape, the perspective is not throughout 
inverted, but portions of the field appear interposed between the eye 
and nearer objects in a singular and somewhat startling manner. 
By arranging the compound rectangular prism so that the optical 
pencil is divided in the plane of vision, instead of vertically, the pseudo- 
scopic effect is almost entirely obviated. 
In constructing the binocular eye-piece, the prisms and arrangement 
of Nachet have been found to answer every condition and requisite of 
binocular vision. The dividing prism being placed, as before stated, at 
the point of crossing of the pencils in the erecting eye-piece, each pencil 
of light will enter the small dividing prism and impinge upon its reflect- 
ing surfaces in a manner similar to that illustrated in the Nachet 
binocular Microscope. The binocular eye-piece has greatly the advantage 
over the other arrangement. For when the prisms are placed in tJie 
binocular body immediately above the objective, their position, in order to 
secure Si proper division of each transmitted pencil, should change with every 
change of objective used — which can be easily provided for in the case of 
low powers, but is rather impracticable with the higher numbers, it 
being very difficult to bring the prisms sufficiently near to the posterior 
combination of the objective. On the contrary, when the binocular 
arrangement is embodied in the eye-piece, the prism being once fixed in 
proper position, as before described, is correctly placed for every power 
of objective, and the eye-piece, thus binocular in form, is as applicable 
through the whole range of powers as if it were monocular. Applied to 
high powers, only one condition would be distinguishingly critical in 
tlie case of the eye-piece — that of the centricity of the central prism. 
The form of erecting eye-piece found most advantageous in this binocular 
adaptation is a duplication of the ordinary Huyghenian negative eye- 
piece, wherein the small dividing prism is very nearly at the eye-hole 
point of such a negative eye-piece as is ordinarily applied in the mono- 
cular Microscope. At a proper distance above this is placed another 
negative eye-piece, in which is formed a second image of the object 
viewed. 
This form of erecting eye-piece gives less extension above the body 
of the Microscope than the positive form, and for that reason is preferred. 
