313 
The residual excitation, or power which remains after ex- 
citation has ceased, is always of the same amount, 130.68, if 
that would have passed half the maximum ; below that it 
bears a continually increasing ratio to the full power till it be- 
comes two-thirds of it. If, while the magnet is in this state, 
a current that would of itself produce the same lift be passed, 
the effect is not doubled, but only increased by one-third. A 
negative current, if powerful, destroys this condition; if feeble, 
only lessens it. 
The least current which he has tried, 0.0008, excites the 
magnet, and even changes its residual magnetism. 
Monpay, June 281TH, 1852. 
THOMAS ROMNEY ROBINSON, D. D., Presipent, 
in the Chair. 
Mr. Berein read a paper on the illumination of objects in 
the microscope. 
‘¢ All who are accustomed to the use of the microscope are ne- 
cessarily aware of the vast improvements which have been effected 
within the last twenty years or little more. Prior to that, the com- 
pound microscope was almost worthless as an instrument of research, 
and inquiries as to minute structures were carried on by means 
of single lenses, or of combinations acting as single lenses: and 
when we look to the works remaining to us of the earlier mi- 
croscopic observers, as Leeuwenhoek, Grew, Malpighi, and others, it 
is truly wonderful what they effected. However, the labour of such 
investigations with such means, or even with the jewel lenses of 
Pritchard, the doublets of Wollaston, or the triplets of Holland, was, 
as every one who has used them well knows, immense, and the injury 
to the sight caused by high powers unfortunately very great and 
enduring. All this, however, has been so amply and ably treated 
VOL. v. 2H 
