47 
- 
lenses. The disturbance, however, may also be owing to the 
brass of the cells, but if so the remedy is the same, namely, 
increasing a little the diameter of the posterior lenses. It is 
true that this involves an increase of their thickness, and of 
course a considerable change in the combination. I would 
also suggest another alteration, in case it be thought de- 
sirable still to make objectives of these extreme apertures ; 
that the anterior surface be concave instead of plane. I do 
not suppose this would much increase the difficulty of the 
work of correction, and in fact No.5 was so constructed. 
To show how much illumination would be gained by this, I 
have computed the fourth column of the Table, supposing the 
curvature such that the final incidence is 75°, and the object 
covered; from which it will be seen, that the difference at 
the limits is nearly the full power of an objective of 60° aper- 
ture, and that it almost compensates for the loss of light due 
to the cover.” 
The Rey. Dr. Todd read a Letter which he had received 
from William H. Harvey, M.D., written in Ceylon in No- 
vember last, giving an account of his botanical and zoological 
collections. He also exhibited a restoration by Joseph H. 
Smith, LL.D., made from a rubbing of an ancient Irish in- 
scribed grave-stone, with an inscription. 
Dr. Petrie made some remarks upon the inscription. 
The Rev. Dr. Todd presented a Walloon tobacco-box, with 
several Flemish inscriptions, from the Rev. William Thomp- 
son, said to have been found on the person of a soldier slain 
in the Battle of the Boyne. 
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