SCIENTIFIC SUMMARY. 
325 
phragm of this construction; from the fact that its opening will not always be 
in the same plane — that is, the smallest cross-section of the space between the 
rollers will not always he equidistant from a plane at right angles to the 
line of sight and passing through the axes of the rollers. With the larger 
opening, this smallest section will he nearest to, and with the smaller, 
farther from, such a plane. In practice, however, this difference is so small 
as to he entirely unimportant, and may even, in some cases, he turned to 
advantage. There are other forms of gradually adjustable stops which have 
been employed with more or less success, hut few, according to Mr. Morton, 
involving so many elements of durability and convenience. 
Browning's Pocket Microscope. — Mr. John Browning has recently turned 
out a pocket microscope, which we have much pleasure in commending 
to the notice of those of our readers contemplating a sea-side tour. Its 
general features are shown in the figure. It has two objectives, 1-inch 
and 2-inch, and a large-field eye-piece. The workmanship, like that of 
all Mr. Browning’s instruments, whether astronomical or general optical, 
is excellent. This instrument — the most portable yet contrived — is made 
with the body in two parts, one sliding into the other ; the outer 
portion also slides down through the opening in the stage, which carries 
the objects. Two legs are hinged at about the centre of the instru- 
ment j the rod or tube, on which the reflecting mirror is fixed, forms a 
third leg. Thus, when the two hinged legs are open, the instrument has a 
firm tripod stand. These legs being opened, the lower part of the body 
drawn up through the stage, and the eye-drawer withdrawn from the body 
to about the same length is all that is required to set up the instrument 
ready for use. The tube spoken of as carrying the reflecting mirror and 
forming one of the legs has a fine screw on the inside, and a milled head at 
the top. This screw gives a fine adjustment. The instrument has an eye- 
