MEMORANDA. 301 



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 suffi- 

 cient difficulty to rank as a severe test-object, while there are 

 many which when balsam-mounted become very satisfac- 

 tory. 



It is certainly no duty of mine to explain why Mr. Wenham 

 has failed in his attempts to resolve the balsam-mounted spe- 

 cimens, 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 microscopes of London, in representing the English 

 glasses as incapable of doing as much. That the English 

 lenses are capable of performing well on balsam-mounted 

 objects of considerable difficulty 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. vii., p. 14 : — " I would here state that in 

 the spring of 1853, I resolved the Greenport Grammatoj)hora 

 [balsam-mounted] unmistakably by a l-4th of an inch objec- 

 tive made by Spencer, and subsequently by a l-4th recently 

 made by Powell of London for Dr. Varnarsdale of New 

 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 difficult than any known to me ; yet I feel no hesitation 

 in challenging 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 GrammatopJiora stricta, Ehr., from 

 Halifax, N. S. This is as much more difficult than the Pro- 

 vidence Grammatophora, as the latter is more difficult than 

 the Greenport specimens. As a supply of the last two 

 varieties has been in London for two years they are probably 

 known to Mr. Wenham, and may have been subjected to 

 experiments by him. That the balsam-mounted specimens 

 of all these objects can be satisfactorily resolved is well 

 known to American observers ; and the following statement 

 given by Judge A. S. Johnson, in vol. xiii., p. 32 of this 

 Journal, is fully confirmatory of my own experience. Speak- 

 ing of a new object-glass of 174^°, made in July, 1851, by 

 Spencer, the following remarks are made : — 



" The light failing us as evening was approaching, we did 

 not try in this way either the Amici test or the Providence 



