ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 867 
of the rods ; and when the latter are driven apart by the effect of the 
spiral spring P, the central parts of the plates are pressed together and 
firmly clamp the crystal placed between them. The opening of the 
clamp is effected by means of the screw q, which presses upon the 
rod o', jointed to the plate n. The crystal is oriented by means of 
the horizontal displacement of the holder, and its rotation about the 
axis at the head of the column, which is regulated by the screw v. 
After the orientation of the crystal, and adjustment of the objective 
and condenser, the burner is brought into the vertical position, and the 
crystal is heated. It is then quickly turned back again, and by means 
of the stop of the screw v the crystal keeps its former orientation. On 
turning back the burner, the flame is at the same time extinguished by 
the rod A acting upon the cock H, and cutting off the gas supply. 
When the burner is brought back again to the vertical position the 
cock is again opened, and the burner re-lighted by means of a small gas 
flame, fed by the tube t, which is kept constantly burning. In the burner 
is a small tube w, communicating with a bellows, which serves either for 
the product 1 on of a blow-pipe flame, or for the cooling of the apparatus. 
The Reflector with the Projection Microscope.* — Mr. G. B. 
Buckton urges the advantages of a catoptric as compared with a dioptric 
arrangement in the following letter : — 
t: The lantern is now used for so many purposes — scientific, photo- 
graphic, and recreative — that any improvement in its construction will 
be acceptable. When we look into this instrument whilst at work we 
must be disappointed at the large quantity of light lost by reflection 
and by dispersion — light which ought to go to the illumination of the 
screen. In the ordinary form of the lantern three lenses of dense glass 
are employed as condensers. Each of these six surfaces reflects and 
scatters the light, and the glass itself is absorbent of its rays. 
The dioptric construction of the projection lantern has been well 
worked out by Messrs. Wright, Newton, Salomons, and others, but the 
catoptric principle, which would eliminate almost entirely these dis- 
advantages, has been scarcely at all studied. 
Although my experiments have been made solely with the limelight 
in various forms, the following remarks may equally apply to light 
given by the electric arc : — If a reflector be used instead of the ordinary 
condenser, it is obvious that the position of the lime cylinder must be 
reversed. This will present no difficulty, for the tube holding the jet 
can be bent into a helical form. The dark image of the lime cylinder 
also will have no more practical disadvantage than is experienced by 
a like image formed by the small plane speculum of the Newtonian 
telescope. 
As to the mirror itself, although a parabolic form is the most correct, 
a spherical surface will be sufficient for mere illuminating purposes, and 
thus expense may be spared in the grinding of the more difficult curve. 
A speculum of from 5 to 7 in. diameter, having a radial curvation of 
from 2.V to 3 in., will grasp a large quantity of light, much more than 
that obtainable frotn the 5-in. condenser usually employed. 
Silver deposited by one of the various reducing processes on the 
* Nature, xlvii. (1892) pp. 54-5. 
