Stonry— Compressed Air Support for Specula. 271 
water under pressure, and suitably to counterpoise the grinder or 
polisher. Perhaps the usual mode of counterpoising will be found 
sufficient, or if necessary a light cell of the new kind may be 
adapted to the grinder or polisher, and the whole counterpoised, 
so as to secure a perfectly clean cut over the surface of the mirror, 
without pressure upon one part more than another. One other 
point seems worth noticing: that if the figure of the mirror, when 
finished, be slightly too hyperbolic or elliptic, it may perhaps be 
brought nearer to the true form by a slight increase or decrease of 
the pressure of the air upon its back, which could be easily secured 
by dividing the front chamber of the cell into two compartments 
with a regulator acted on by a spring between them. 
It may perhaps be well to mention that the pressure of the air 
to be stored in the gasometer needs only to be of moderate amount, 
and can be easily provided by an automatic water-dropping 
arrangement. ‘Thus to support a glass mirror five inches thick, a 
maximum pressure of only one inch of mercury is requisite—‘.e. 
one-thirtieth of an atmosphere. 
On the whole there seems reason to hope that reflectors may, 
with advantage, be substituted for refractors in photographing the 
heavens, for spectroscopic work, and in some instruments of preci- 
sion; that the instruments will be cheaper, and the work produced 
by them distinctly better. 
SCIEN. PROC. B.D.S., VOL. VIII., PART III. U 
