102 
CORRESPONDENCE. 
their object-glasses can be either popularly, easily, or accurately 
estimated. 
Under these circumstances opticians can hardly be expected to 
devote themselves with ardour to objectives of this sort. The finest 
glass ever made would have but a limited sale, and probably would be 
condemned by the public because it could not perform on a diatom so 
well as other known glasses of even moderate excellence. 
Still it is most desirable that makers should be encouraged to 
throw their utmost skill into the manufacture of glasses best fitted 
for scientific work. 
With this view I would suggest that the Eoyal Microscopical 
Society should appoint a committee to settle upon a standard for 
physiological glasses. That the Society should offer a gold medal 
every third year or oftener for such glasses, the same committee to 
judge of those sent in for competition. The prize should be open 
to all the world. 
I have no doubt at all but that our great makers would add new 
laurels to those they have so long and with such good right worn. 
But whether that were so or not in the first instance, we may be sure 
they would accept no defeat ; that they would be stirred up to new 
efforts, and that science would thereby be the great gainer. 
The glass bearing off the Society’s medal would be stamped in the 
public mind as the best of its kind ; and while it would be a real and 
immediate boon to science, it would by its reputation teach the in- 
telligent amateur to look to a more instructive field than that presented 
by mere diatoms, for the interest and pleasure he derives from his 
microscope. 
Dear Sir, yours truly, 
E. Branwell, F.E.M.S. 
The Bucephalus parasitic on the Fresh-water Mussel. 
To the Editor of the ‘ Monthly Microscopical Journal.' 
Stoke-upon-Trent, July 10 , 1875 . 
Sir, — In the account of the ‘ Proceedings of the Eoyal Micro- 
scopical Society ’ appended to your July number, a doubt is expressed 
whether an instance has ever occurred of the genus Bucephalus being 
found parasitical on the fresh-water mussel. Forty years back the 
writer of this communication found them in the ovary of that animal 
(Anodonta), and figured them in the ‘ Trans. Zool. Soc.,’ vol. ii. 
pi. xviii. In the same year he found in the ovary of another 
Anodonta, in vast numbers, the mother-cells of a Distomus, each cell 
containing several individuals. This is figured in the same plate as 
the Bucephalus. These Distomi, when excluded, are probably the 
same which are found in the mantle or outer tunic of mussels 
generally, and constitute the ordinary nuclei of pearls. 
Your obedient servant, 
Eobert Garner. 
