28 Prof. Bailey’s reply to Mr. Wenham. 
the actual magnetization of its substance could not be the reverse 
of that of soft iron, and that the surrounding medium (whether 
it be air or what we habitually but falsely call vacuum) must ex- 
perience magnetization similar to that of iron in the same position, 
and greater in degree than that of the bismuth. According to 
substance less magnetizable than air. Prof. Thomson further re- 
marked, that he had not perfect confidence in the truth of this 
couelnsion, as one of the assumptions on which the reasoning 
as founded admitted of doubt; but he had no doubt whatever 
of t ne Rests polarity of bismuth, however occasioned, being 
the reverse of that of iron. He concluded by expressing com- 
prt ceiuasaens with Prof. Tyndall on this point, and admiration 
of the remarkable combination of powerful and delicate apparatus, 
and the beautiful and well planned experiments by which Prof. 
Tyndall had so successfully demonstrated the antithesis between 
iron and bismuth to the Meeting. 
Art. V.—Reply to some remarks by W. H. Wenham, and Nolice 
of a new locality of a Microscopic Test-object ; by Prof. J. W. 
Baitey, U.S. Military Academy, West Point. 
In an article by W. H. Wenham, Fsq., of London, published 
in the Quarterly Jousila of ane A Science for July, 1854, 
I have noticed the following ap oig 
“These experiments [made by Me Wenham] will readily 
account for the difficulty of discovering the markings or structure 
of a severe test when mounted in balsam ; for as thus seen it may 
be inferred that no aperture exceeding 85° can be made to bear 
upon it, and this is even supposing that the largest aperture 
object-glass that has ever been constructed is used. Such being 
the case I am somewhat puzzled at an announcement that ap- 
pears to contradict this fact, coming from one that must be con- 
sidered as authority in these matters. I refer to Professor Bailey, - 
who, in a letter addressed to Matthew Marshall, Esq., dated Janu- 
ary 20th, 1852, first speaks of an American object-glass of very 
large aperture (1724°) and its performance on the most difficult 
tests known, aud then proceeds to say ‘In all these cases (and in 
fact whenever I allude to a test-object) 1 mean the balsam mounted 
he dry shells I never use as tests.’ is assertion 
seems to m me to be extraordinary, and very like saying that an aper- 
ture of 85° or 90° will do every thing that is required. I have 
invariably found that when very difficult tests are mounted in 
balsam I discover the markings, and certainly the reasons 
unt Fit it. Iti is to” be es that the Ameri- 
oe 
