Testing Object-glasses. By Br. Boyston-Pigott. 151 
to an open window disclosing a lovely foreground. Eighth O.Gr. 
A low eye-piece : power 400. 
Besult. Landscape dark and hazy. The deficiency of light was 
most remarJcahle. The same power was now got from a half-inch 
and D eye-piece. 
New result. An exquisite picture brilliantly lit up, even the 
glittering foliage twinkling in the sunbeams, and the garden details 
were marvellously displayed. This difference is truly surprising. 
Increased light and suferh definition with diminished aj)erture 
and same power. 
To those who have not yet tried this method, and are desirous 
of becoming acquainted with the peculiar effects of over or under 
correcting their glasses as far as their construction is capable, I 
may be permitted to recommend the following commencement with 
these experiments : 
A. Experiment for spherical aberration. Kemove the front 
glass of the observing objective. Examine the miniatures of the 
illuminated globules. 
B. Bemove the internal glasses and replace the front only. 
C. Eeplace all the glasses and close up the screw-collar for the 
mark " covered " if there be one. 
D. Open the collar for "uncovered" position. At the last 
point for the first time the miniatures will begin to more nearly 
resemble their original. 
The various changes of the appearances of the miniature, to 
the student of this department of optics, form a most interesting 
study. 
These phenomena, varying from a mild brilliance to a gorgeous 
splendour, according to the quality of the light, and its mode of 
exhibition, may be greatly varied by the reverse process, viz. 
changing the lenses of the wdniatureAoimmg glasses, just as done 
with the observing. Some of old Pritchard's exquisitely minute 
lenses mounted by him give very charming miniatures. But 
these form no test of the quality of the objective in use, because 
their aperture is extremely limited. 
I now may be allowed to describe what has not before appeared 
before the Society — the singularly beautiful phenomena displayed 
by miniatures of sun-lit mercurial globules. 
It is well known that the surface of minute globules of mercury 
becomes more nearly spherical as they diminish in weight. The 
law of the curvature of these surfaces, dependent upon the specific 
attraction of mercury, has been investigated by Professor Bash- 
forth, though not yet published ; and according to this law they 
seem incapable of forming a perfect image by reflexion. But 
under direct illumination (not that wild bull's-eye side illumination 
of the globule hitherto employed) a minute spectrum of the sun 
