340 
of possible paths exist between the points (#,, r,), and (2,, r,), 
which are therefore conjugate foci, 
Differentiating V with respect to x, and x, we obtain 
stl ME cana etl alba th ea 
da, div, D > Ge iar a,)(7, me a,) ahs 
D is the quantity in Cotes’ Theorems which Dr Smith calls 
the Apparent Distance, or the distance at which the object 
must be placed that it may subtend the same angle as when 
viewed through the instrument. 
+ &e., 
We have also 
dD aD 
Files ae eee aos SP; ar Te ae ic 
1 2 
March 16, 1874. 
The PRESIDENT (PROFESSOR BABINGTON) in the Chair. 
Communications were made to the Society : 
(1) On the use of the term Endothelium. 
By Dr Micuagn Foster. 
In this paper it was shown that the term “endothelium” has 
been recently introduced into histology: and the use of it has 
rapidly become common if not general. The speedy acceptance 
of a new term may in many cases, though not all, be taken as 
an indication that something of the kind was wanted: and the 
already frequent use of “endothelium,” both by Continental 
and English Histologists, would seem to shew the need of some 
other phrase besides “epithelium.” Nevertheless there are 
cogent reasons why the new term should not be allowed to take 
farther root. 
In the first place, its etymology is of the most grotesque 
kind. This is of course an objection of secondary value, but 
has still some weight. When a term has once come into daily 
