Prof. Bailey’s reply to Mr. Wenham. 29 
object-glasses, that will render a smaller amount of aperture ser- 
viceable ; but however this may be, I think that Professor Bailey’s 
statement requires some explanation.” —Journ. Mic. Science, July, 
It is apparent from the above that Mr. Wenham has convinced 
himself, both by “reasons” and experiment, that I ought not to 
have seen the markings on delicate test-ohjects when mounted in 
balsam ; and that as he invariabl y found that he could not discover 
these markings, therefore some new and peculiar principle in ob- 
ject-glasses must have been discovered to account for the success 
of American opticians. In answer to this I would state that both 
in print, as well as in private letters I stand fully committed to 
the statement that I can resolve the most difficult tests known 
even when mounted in balsam. Iu 1849 I stated in this Journal, 
Vol. vii, p. 268, that “the resolntion of these tests mounted dry is 
so much easier than when in balsam, that objects thus mounted are 
of little value in testing the powers of lenses, although they may 
answer well when the end is to make out the real structure of 
the object itself.” In fact I have up to this time met with no 
object which, when mounted dry presents sufficient difficulty to 
rank as a severe test-object, while there are many which when 
balsam-mounted become very satisfactory. 
It is certainly no duty of mine to explain why Mr. Wenham 
has failed in his attempts to resolve the balsam-mounted speci- 
mens, particularly as the resolution of such tests is a matter of 
every-day amusement with microscopists in this country, and I 
believe Mr. Wenham does injustice to the microscopists and mic- 
roscopes of London, in representing the English glasses as inca- 
pable of doing as much. That the English lenses are capable of 
performing well on balsam-mounted objects of considerable diffi- 
culty I know by my own trials, some of which are referred to in 
the following paragraph from a paper recently published in the 
Smithsonian Contributions to Knowledge, vol. vit, p. | eet 
would here state that in the spring of 1853 I resolved the Green- 
port Grammatophora [balsam-mounted] unmistakably by a 4 of 
an inch objective made by Spencer, and subsequently by a Z re- 
cently made by Powell of London for Dr. Vanarsdale of N. York.” 
As Mr. Wenham does not mention the names of the test- 
objects employed by him, I cannot say that they may not be more 
‘ficult than any known to me; yet I feel no hesitation in chal- 
lenging him to produce an object resolvable when dry, which I 
Cannot resolve when balsam-mounted. I will also state that I, at 
Present know of no test-object more difficult than a supposed 
Variety of Grammatophora stricta, Ehr., from Halifax, N.S. This 
1S a8 Much more difficult than the Providence Grammatophora, as 
the latter is more difficult than the Greenport specimens. Asa 
Supply of the last two varieties has been in London for two years 
they are probably known to Mr. Wenham and may have been 
