NOTES AND QUERIES. 75 

two straight slips of brass three inches long, one of which is fixed 
at right angles to one slot, and the other is moveable in the second 
slot. These evidently centre quite accurately for breadth, and 
longitudinal centering is secured by fixing a pin in the upper 
surface of the table 13 inches from the centre.—A. J. D. 
[We cannot see how a slide can be centered by this arrange- 
ment.—ED. | 
PHOTO-MICROGRAPHY.—The sketch of the shutter on page 48, 
Vol. II. of THE NorTHERN Microscopist may be used for ascer- 
taining the correct exposure as well as the difference in adjustment. 
It is more correct in principle for ascertaining the exact exposure 
than the usual method of drawing out the shutter a little farther 
each time, on account of its working from the centre. 
Your “ aperture shutter” will be of great use to the Photo-micro- 
grapher. When I first saw Collins’ graduating diaphragm, I 
thought of its use in Photo-micrography if it could be applied to 
the objective, but was under the impression that.it would require a 
new mount for each objective, as I presumed the diaphragm was 
fixed at the best working distance. I now find it useful in the form 
you figure it in the Journal for January.— W. Shipperbottom. 
Diatoms In THAMES Mup.—An observation showing the value 
of the Microscope in settling vexed questions may be found in the 
Proceedings of the Holmesdale Natural History Club, wherein it 
is proved beyond doubt that the mud-banks forming in the river 
are due to the discharge of matters from the sewage out-fall. 
Mud from different localities was microscopically examined :— 
At half-a-mile above Teddington Lock sixty-six fresh water diatoms 
were found, but no salt-water species ; at one mile below the Lock 
fifty-four fresh-water forms were found. At Kew fifty-two fresh- 
water, and thirty-seven marine forms; at Blackwall thirty-nine 
fresh-water, and forty-five marine; while in the Estuary of the 
Thames only nine fresh-water species were found, accompanying 
sixty salt-water species. . 
STANDARD GAUGES FOR EvE-PIECES.—The Committee appointed 
by the Council in October last, to consider the question of standard 
gauges for eye-pieces and substages, duly presented their report, 
which was thereupon ordered to be printed and circulated amongst 
the members of the Council, and is now under consideration.— 
PaeMI ASG lebi et Soe: 
THE APERTURE SHUTTER.—Mr. Chas. Collins, it appears, has sent 
one of his “aperture shutters” to the Royal Microscopical Society 
for exhibition. At the meeting held on the 8th inst., it was referred 
to, and the credit given to Dr. Royston Pigott. We here claim the 
apparatus for Mr. J. B. Dancer, of Manchester, and shall allude to 
it in a special article in our next issue. 
