Dr. Carpenter, on Stereoscopic Binoculars . 
107 
As an ordinary working instrument, however, this improved 
Nachet Binocular can scarcely be said to possess any point 
of superiority to the Wenham; whilst it must be regarded 
as inferior in the following particulars : — First, that as the 
uninterrupted half of the cone of rays (when the interposed 
prism is adjusted for Stereoscopic vision) has to pass through 
the two plane surfaces of the prism, a certain loss of light 
and deterioration of the picture are necessarily involved ; 
whilst, as the interrupted half of the cone of rays has to pass 
through four surfaces, the picture formed by it is yet more 
unfavorably affected; and second, that as power of motion must 
be given to both prisms — to a, for the reversal of the images, 
and to B for the adjustment of the distance between the two 
bodies — a greater liability to derangement results than in 
the simpler construction of Mr. Wenham.* 
Nachet’s Binocular Magnifier. — Though the Author can 
testify to the fidelity of the effect of relief obtainable by 
Nachet’ s Binocular Magnifier, adapted to Beck’s Dissecting 
Microscope. 
the Binocular arrangement adapted by Mr. B. Beck 
is made both suddenly and completely. In more complex and less familiar 
forms, on the other hand, the conversion frequently requires time ; being 
often partial in the first instance, and only gradually becoming complete. 
And there are some objects which resist conversion altogether, the -only 
effect being a confusion of the two images. 
* This arrangement, like Mr. Wenham’s, can be adapted to any existing 
Microscope ; and it seems peculiarly suitable to those ot French or German 
construction, in which the body is much shorter than in the ordinary English 
models. For in the application of the Wenham arrangement to a short 
Microscope, the requisite distance between the eye-glasses of its two bodies 
