4 Beck^ on Two Neio Forms of Reversible Compressors. 
view are unequal, and the space included in a small circle only is 
stereoscopic: but this result is no worse than the ordinary dis- 
advantage of smallness of field common to all binocular single 
microscopes of moderate power ; whereas the advantages over 
the quadruple reflections are, much less loss of light, only half 
the cost, and great facility in turning the prisms out of the 
way when one eye only is used. 
The mounting required by this binocular arrangement 
is as follows : — To a semi-cylindrical tube are attached 
two i_ prisms; one is a fixture, but the other will slide 
in or out to suit the distances of different eyes ; the two 
together are connected with a second arm, which fits im- 
mediately above the first arm, carrying the magnifier, 
and if both eyes are used the right-hand prism should 
exactly cover one half of the magnifying lens. The second 
arm is stopped when it reaches this position, but it can be 
turned out of the way to any extent in the other direction, 
and the fittings of the prisms allow of their easy removal when 
they require cleaning. 
As to the value of binocular single magnifiers, I prefer 
quoting the words of Professor Riddell to giving any opinion 
of my own ; he says, " I regard the binocular magnifier as 
supplying a great desideratum to large classes of persons pur- 
suing a great diversity of calling. The effects, so often pre- 
judicial to vision, of inordinately using one eye, are thus 
avoided, and a perfectly natural relief or definition of bodies 
in depth, as well as in extension, is thus attained.^' 
Description of Two New Forms of Reversible 
Compressors. By Richard Beck. 
(Read Oct. 14th, 18G3.) 
Both these pieces of apparatus are contrived to meet the 
following wants : — A parallel and equal compression, with an 
immediate action up or down ; the means of turning the 
compressor over, so as to see both sides of an object without 
disturbing it ; the use of thin glass only, either at top or bot- 
tom ; and, lastly, such restrictions in the thickness of the 
brass work as will permit the use of the achromatic condenser 
or the parabolic-reflector. 
I am indebted to Mr. Slack for a suggestion which forms 
the basis of the construction in each case. He proposed 
that two oblong pieces of thin glass should each be secured 
