On the Navicula Spencerii. 267 



ever could make nothing of it. * * * We had some of the 

 finest glasses of Smith, Ross and Powell in our examination, and 

 I am bound to state that at present the result is most unsatisfac- 

 tory." A postscript to the same letter says, " Since the above 

 was written, I have made several efforts to get at the markings on 

 the N. Spencerii, but without success." A subsequent letter da- 

 ted June 2d, 1848, says, " Still however, I can make nothing of 

 your N. Spencerii, although I have employed one of Powell's 

 best glasses with the power of at least 1100." 



Knowing by my own experience that the difficulty which my 

 London friends met with, was due to their not employing the 

 much greater obliquity of light which this object requires than 

 any other test previously known, I again wrote to London and 

 sent particular directions concerning the mode of illumination 

 used by Spencer and myself, at the same time expressing my be- 

 lief that, (as was really the case,) before my letter could reach 

 London the difficulty would be overcome. A letter received by 

 me about the time/ from Manchester, England, dated July 27, 

 1848, is as frank in its acknowledgment of the difficulty of re- 

 solving the N. Spencerii, as those from London above alluded 



to. It says, "You have indeed fixed us all now; D , and 



myself have given the thing up in despair, and must confess our- 

 selves thoroughly beaten by your Yankee back-woodsman." 



The next letter from London announced, as I expected it 

 would, the success of the London microscopists in resolving the 

 test. It is dated July 28, 1848, and says, u I have had an oppor- 

 tunity of submitting your specimens to a very careful examina- 

 tion by the best glasses of Ross, Powell and Smith, our three 

 best makers, and by oblique light we are enabled to resolve them 

 most unmistakeably. But all our efforts to detect the markings 



by direct light proved fruitless." 



May 



some time in July, the Navicula Spencerii remained in London 

 and not a line, much less dots or perforations, could be seen upon 

 J t ; although Spencer had resolved two sets of lines upon it by 

 means of his lenses, mounted as he informs me in a hand tube 

 and without the aid of achromatic condensors, adjusting screws 

 °r even a stage to support the glass slide, a feat, by the way, of 

 the practicability of which I have convinced myself, but which 

 is unparalleled in the history of optics. 



It would be unjust to our English friends, if I did not now pro- 

 ceed to show what they developed upon the test in their subse- 

 quent trials. And it will presently be shown that Spencer has 

 a t least kept pace with them without any previous knowledge of 

 their progress. A letter from London, dated August 18, IS 18, says, 

 "the N. Spencerii is certainly the most trying test I have yet met 

 with. * # # [ have resolved the cross lines upon it mounted in 



