84 Reply to Mr. De la Rue’s remarks on the N. Spencerii. 
To Mr. Spencer I am indebted for the knowledge of two test- 
objects of far greater difficulty than the N. Spencerii. The first 
made by Ross of London and recently imported by Mr. Cole of 
Salem. The Gt. subtilissima occurs along our whole coast from 
Massachusetts to Florida. In form it resembles the G. marina of 
authors, but is more delicately marked than my European speci- 
mens of that species. The specimens with the finest markings 
I found growing upon the Polysiphonia Olneyii of Harvey, near 
R.. I. 
Providence, 
resolve them even when mounted in balsam. I think that when 
r. De la Rue sees these objects, his doubts “ whether the phys- 
ical properties of light do not put a natural limit to our resolving 
lines so close as the ;5;'s;;th and 5,,';5;th of an inch” will be 
removed. 
Note.—Since the above was in type, I have been informed by — 
Mr. Spencer, that he considers many of the specimens from Prov-. 
idence as more difficult than any of the specimens of Amici’s test. 
The Grammatophora from Greenport, Long Island, is decidedl 
