Wenham on Binocular Vision. 
5 
It has also been proposed to bisect the whole combination, 
of which our best objectives are composed, and separate the 
semi-lenses a sufficient distance asunder to obtain the effect 
of stereoscopic vision, each half being made to serve the 
purpose of a distinct combination ; but this, I believe, would 
not answer at all, for if we escaped the total destruction of 
the object-glass during the operation of sawing it through, we 
should render it useless for all the ordinary purposes of inves- 
tigation, and also because any separation of the semi-lenses 
is quite unnecessary; for the angle of aperture of all the 
object-glasses by our best makers now exceeds that which is 
requisite for obtaining stereoscopic vision ; and the methods 
that I have now to explain refer to the principle of obtaining 
two images of the object through the same object-glass, which 
is in all cases of the usual construction. 
In the last * Quarterly Journal of Microscopical Science ' 
there appeared a notice of a binocular microscope by J. L. Rid- 
dell, from Silliman's Journal. Fig. 4. 
According to his description, 
fig. 4 will represent the ar- 
rangement ; a is the objective 
provided at the back with the 
usual stops. The pencil of 
rays emergent from the ob- 
ject-glass is bisected and re- 
flected in opposite directions, 
by means of the internal 
surfaces of the rectangular 
prisms h b, which surfaces 
are inclined at an angle of 
45°. The rays are again re- 
flected in a vertical direction, 
by means of two similar 
prisms, c c, the distance be- 
tween which must be regu- 
lated by the position of the 
eyes. The last prisms must 
be placed upon a lower level, 
as from the direction in which 
the rays are incident upon 
the first reflecting surfaces of the prisms b b, they have a down- 
ward tendency. The rays, after crossing each other, are received 
by two Huygenian eye-pieces, d d. In the diagram I have 
shown the prisms no larger than necessary for collecting all 
the rays from any of the object-glasses to be used ; but it 
must be evident that Mr. Riddell makes use of prisms of 
