880 Astronomical Society. 
torily) for the long interval elapsed from the first reception of 
the disc to the final completion of the object-glass. 
In the state in which it has been submitted unreservedly to 
their inspection, at Mr. Tulley’s house at Islington, mounted 
in a temporary wooden tube, and on a stand of very conve- ; 
nient construction for astronomical uses, its clear aperture is 
six inches and eight tenths, and its performance has proved in 
the highest degree satisfactory. It has been tried by us on 
various objects, both by day and by night; among the latter, 
the planets Jupiter and Saturn, several of the most delicate 
and difficult double stars, such as Polaris, y Leonis, ¢ Cancri, 
a Leonis, &c. as well as some of the small resolvable nebulz 
in the constellation Virgo; severe tests these of the perform- 
ance of a telescope, under magnifying powers from 200 to 
700. 
The examination of a bright object on a dark ground, as a 
card by day-light, or Jupiter by night, with high magnifying 
powers, affords, as is well known, the severest test of the perfect 
achromaticity of a telescope, by the production of green and 
purple borders about their edges in the contrary case. The 
telescope in question bears these tests remarkably well, and is 
certainly more achromatic than usual; a circumstance de- 
pending not merely on the nice adjustment of the foci, but on 
the quality of the flint glass mainly. This might not have 
been expected (according to a remark of Dr. Brewster) from 
the high refractive and dispersive power of the glass, but the 
fact is undoubted. 
The destruction of the aberration of sphericity in an object- 
glass, when thoroughly accomplished, even with the best ma- _ 
terials, is the strongest proof of the goodness of its workman- 
ship; but except the materials be good, no excellence of work- 
manship will destroy that irradiation which surrounds the 
image-of a star with lines of light darting from it as a centre, 
and which fills the field with loose dispersed light. The ob- 
ject-glass in question is perfectly free from the latter defect, 
and almost entirely from the former. The rudiments of rays 
may, indeed, be traced in interruptions of the regular con- 
tour of the rings which surround the spurious discs of large 
stars, and which arise from the interference of the rays grazing 
the edge of the aperture. Portions of these rings are wanting 
or very faint, and other portions somewhat stronger; so that 
in some directions the outlines of rings of several orders may 
be traced, in others only those of the first and second. This 
defect was distinctly perceived in the image of y Leonis, with 
a power of 220, giving it the appearance of having an ex- 
cessively faint small star, almost close to the large one of the 
double 
