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. 



Monday, June 28th, 1852. 



THOMAS ROMNEY ROBINSON, D. D., President, 

 in the Chair. 



Mr. Bergin 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. 2 H 



