28 Mr. Swainson on the Characters 
injured by having such an apparatus to it as I have recommended, 
even though the eye-tube were only of the common length; it is 
certain it could do no harm at Jeast. I think I can, moreover, with 
confidence assert that increasing the distance between the small 
metal and the eye-glass, for the purpose of applying stops, will not 
be found to make the least sensible difference in the performance 
of an instrument, as far as the figure of the small metal is con- 
cerned, provided it is of the standard goodness*; if but imperfect 
such an alteration will evidently try it more, and this will be shewn 
by examining a double star which will probably vary slightly 
in the distance at which the stars appear separated, (ceteris pari- 
bus,) according as the eye-glass approximates to, or recedes from, 
an imperfect diagonal. To conclude, as an Herschelian telescope 
is nothing but a Newtonian, used without the interposition of a 
small metal reflector, whatever has been said of the latter, will 
equally apply to it, and the same principle in the eye-tube and ad- 
jJustment, will for the same reasons be equally adapted to both, 
though the manner of execution will be different; I have, however, 
made no experiments on this kind of telescope. 
[The portion of this paper relating to Microscopes is reserved 
for our next Number. ] 
eee ener teil 
Arr. IV. The Characters of several New Shells, belonging 
to the Linnean Volute, with a few Observations on the pre- 
sent State of Conchology. By William Swainson, Esq., 
F.R. and L.S. 
Tue study of conchology has now become so general, or, if I may 
be allowed the term, so fashionable, that the number of elemen- 
tary works is truly surprising. The new systems of the French 
conchologists have been translated, explained, and advocated, in 
various publications; while the admirers of the Linnzan method, 
* It is evident that if the diagonal metal were quite perfect, it could make no 
difference at what distance the eye-glass was placed from it; if decidedly im- 
perfect, it is no less plain that the nearer the eye-glass is placed to it the bet- 
ter, because the less of the edges will be called into action, which will of 
course be the worst part. 
