Reply to Mr. De la Rue’s remarks on the N. Spencerii. 83 
i. e., our micrometers must differ. My own is an English one 
divided to ;;';,ths of an inch. Mr. De la Rue states that his is 
accurate I cannot now determine, and must leave it to future ob- 
servation, I will remark however that Mr. Spencer of Canestota 
in some measurements which he has recently made, obtained re- 
sults intermediate between those obtained by Mr. De la Rue and 
myself, making the distance between the transverse rows about 
yorors of an inch. 
In his remarks, Mr. De la Rue emphatically states that at the 
time of its arrival in London, the markings on the N. Spencer 
shown as lines were not a difficult test for the object-glasses of 
the London observers. 
I leave it for others to reconcile this statement with the quo- 
tation which I gave from a letter from a London correspondent, 
which I will now requote, as follows :— 
“The evening I received your package there happened to bea 
small gathering of our microscopic friends. Your slide with the 
Navicula Spencerii underwent a long examination. We however 
could make nothing of it. * * * We had some of the best glasses 
of Smith, Ross and Powell in our examination, and I am boun 
to state that at present the result is most unsatisfactory.” 
It appears from the above that it was not Mr. Marshall’s glass 
alone which in the earlier trials of this test in London failed to 
resolve it. The fact was that all observers found it a difficult 
object until they became aware of the great obliquity of light 
required, and most observers admitted it to be a far more difficult 
test than any at that time known. Mr. De la Rue appears to be 
the only one who found no difficulty in his first attempts to see 
this as a lined object, probably owing to the fact that the difficul- 
e 
all his skill it was only “after working about an hour that the 
dry specimen was resolved most satisfacto 
