24 W. De la Rue on the Navicula Spencerii. 
cee ; the markings of some I hold to be depressions, but those of 
N. Spencerii to be wart-like prominences. 
With this correction, | proceed to answer Prof. Bailey, first pre- 
mising, that 1 bear most willing testimony to the strong feeling 
of justice and absence of national prejudice, which pervades all 
the letters written by that gentleman to my friend, Mr. Marshall, 
whenever he speaks of the labors of English opticians ; likewise 
the absence of any wish to eulogize Mr. Spencer’s productions, 
at the expense of others. It is not, perhaps now, an unfitting 
occasion, to state that English microscopists have, during a long 
een much indebted to the kindness and zeal of Prof. 
Bailey, furnishing them, through the medium of the gentlemen 
favored by his correspondence, with any new objects, his inves- 
tigations might have brought to light. I mention this, in order to 
show that the highest esteem is entertained by the English micro- 
scopist for Prof. Bailey, and that it is unlikely that he would be 
charged directly or indirectly, “ with underrating the Euglish mi- 
roscopes,” or with any wish to “ overrate the merits of Mr. Spen- 
cer’s, or the difficulties of N. Spencerii as a test object ;’? more 
especially by those who have, like myself, had an opportunity of 
seeing portions of his correspondence relating to the subject. 
As [ hold, that the difficulty of the N. Spencerii has been un- 
willingly overrated, if the exhibition of its markings, as mere lines, 
be the only test of the powers of an oct a I should be 
wanting in candor, if I did not record my o 
Not having taken part in the anvidapoiderits quoted by Prof. 
Bailey, I do not, in any way, hold myself answerable for the oppo- 
site impressions, it may tend to convey ; it is not however a ques- 
tion, of what this or that observer is capable of showing with a 
given object glass, but what the ‘glass can really be made to do. 
With one exception, I believe that none of the object glasses, spo- 
ken of by Prof. Bailey’s London correspondent, were incapable of 
resolving the Navicula; the fairness of which reference I hope to 
establish. The exception [ allude to, is Mr. Marshall’s own glass, 
which, on careful examination, we found to be defective ; and, 
in consequence, it was placed in the hands of the maker. 
I will now as briefly as possible, state my acquaintance with 
the N. Spencerii, in order to show that any difficulty in resolving 
— posi was not due to the object glasses possessed by th 
“ Lon 
It was a A batquentiy to the rig dated 2nd June, 1848, 
quoted by Prof. Bailey, that I heard of the Spencerii, when Mr. 
object from America, which he would be glad of an opportunity 
of examining with me, as he could make nothing of it; an mn Op yor 
Inity soon presented itself and we met at my house. — 
| 
: 
