208 
POPULAR SCIENCE REVIEW. 
complete the connection. The motion of the micrometer-screw now com- 
mimicates no tremor to the microscope, and all difficulty in counting the 
lines seen (whether real or spurious) disappears.” Still better than this 
is the following method : — The microscope being set up in a dark room, as 
though to take a photograph, and the eye-piece being removed, the image 
of the band to be counted is received on a piece of plate-glass in the plate- 
holder, and viewed with a focussing-glass, on the field-lens of which a 
black point is marked ; as the focussing-glass is moved on the plate from 
side to side, the black point is moved from line to line. The lines may 
thus be counted with as much ease and precision as though they were large 
enough to be touched with the finger . — Monthly Mia'oscopical Journal^ 
Februaiy. 
Mow to Construct Object-Glasses . — Those who wish to know not only the 
scientific principles on which the best object-glasses are made, but the most 
satisfactory methods of manufacture, should read the excellent papers of Mr. 
F. H. Wenham which are now appearing in the Monthly Microscopical 
Journal. 
The Quekett Club Soiree was held at University College on the evening 
of March 12. It was an immense success. More than 1,500 persons were 
present, and the 200 microscopes on the table were provided with objects of 
more than usual interest. ' 
A new Growing Slide, which is simple and convenient, is described by Mr. 
C. J. Muller in the Monthly Microscopical Journal of March. Any ordinary 
glass-slide is pierced with a minute hole, at about three-tenths of an inch 
from the centre on one side. When an object under investigation is put 
upon it immersed in water, the thin glass cover is so placed as to include 
this hole, which may be near the margin of the disc. When it is desired to 
keep the specimen moist while off the stage of the microscope, the slide is placed 
in the undermentioned piece of apparatus j viz., a fiat trough 7 inches long, 
2^ inches wide, with straight sides f of an inch high. In this the slide is 
placed, object uppermost, with one end (that nearest the hole) resting against 
the bottom of the vessel on one side, and the other end resting upon the edge 
of it. Sufficient water is put into the vessel to admit of the liquid reaching 
within a quarter or half an inch of the glass cover on the uppermost side, 
when it will be found that, by capillary attraction, the water on the under- 
side reaches beyond the centre of the slide, and consequently beyond the 
hole with which it is pierced. In this state the object will remain moist so 
long as the trough contains a sufficient quantity of water. When required 
to be placed on the stage of the microscope, the water is easily wiped off 
without disturbing the object. 
Obtaining Diatoms from Guano . — In a paper which he read before the 
Natural History Society of Armagh on January 17, Dr. Lewis 13. Mills said 
that in the guano usually to be liad in this country the diatoms form a very 
small percentage of the entire mass, and to prepare the deposit for mounting 
in the rough, according to the usual process, would generally give very poor 
resulte, and discourage all except those well skilled in manipulation. How- 
ever, the most unproductive samples of guano contain some diatoms, and fair 
slides may be prepared from the material, if the process of selection be 
lidopted in their preparation. For this process, it is not needful to use more 
