Beck^ on the Universal Achromatic Microscope. 13 
ifi attached to an outside sliding or tube,, the inner tube being 
screwed beneath the stage^ and almost opposite a side con- 
denser (u) is fixed for the illumination of opaque objects ; it is 
provided with ball-and-socket joints^ which afford any neces- 
sary movement^ and also the means of turning it out of the 
way when not in use. 
The object-glasses are two in number^ one inch^ and a quarter 
inch focusj and of the respective apertures of 20 and 
75 degrees ; these^ when combined with the two eyepieces 
give the four linear powers of 60^ 90^ 240^ 360 ; the 
higher power has no adjustment for variations in the thin 
glass or other media when interposed between its front lens 
and the object^ but it is corrected for a piece of glass '008 
thick^ and for objects covered with glass of such measurement 
its definition will be best. 
The eye-pieces are small in diameter^ but are of a construc- 
tion (we believe introduced by Kelner) which gives a flat^ 
and for their size a large^ field of view. Their chief fault 
will^ we believe^ prove a general advantage ; any dust or 
moisture upon the field-lens is so annoyingly apparent^ from 
its being in the focus of the eye-lens^ that those] who use 
this form will be compelled to wipe the lenses frequently ; 
and not only this^ but they will soon learn the necessity for 
the constant examination and the occasional cleaning of 
every surface of glass that they have about their micro- 
scopes. 
With merely enumerating a pair of forceps (o)^ a small pair 
of pliers (v)^ a glass plate^ with a ledge (w)^ and an upright ma- 
hogany case, we sum up the achromatic microscope we shall 
supply for five guineas. 
Although, up to this point, the instrument may be consi- 
dered as complete and probably sufficient for many ob- 
servers, yet we are quite sure, if any one who possessed such 
a stand were to find that, as his interest in microscopic re- 
search increased, he were unable to make additions to the power 
and to the capabilities of his instrument, the investment even 
of five guineas would become a source of regret or a check, 
perhaps to his enterprise, whilst no credit would be due to the 
manufacturer. We hope, however, to obtain the most opposite 
results from the following additions, which can be made to 
those parts we have already described. 
In the first place a lower, an intermediate, or a higher 
magnifying power than those mentioned may be obtained by 
the addition of a 2 in., Hn., or ^ in., the respective angles of 
aperture of which are 9, 45, and 85 degrees, and their mag- 
nifying powers with the two eyepieces mentioned 30, 45, 
