26 W. Dela Rue on the Navicula Spencerii. 
whether the N. Spencerii had similar dot-like markings ; to deter- 
mine this was a matter of greater difficulty than to resolve them 
into cross lines; but, after working for about an me the dry 
specimen was most satisfactorily resolved; and the dots s exhib- 
ited on that occasion we did not succeed so well in : ae in 
the markings of the balsamed specimen. Direct illumination 
was used on this occasion, the illuminator being a 4th inch object 
glass of 60°. 
It appears that Mr. Marshall communicated in his letter of the 
18th August, 1849, quoted by Prof. Bailey, that the dry specimen 
had been resolved into dots, and that ere long, he hoped to report 
the resolution of the balsamed specimen ; this has been since re- 
peatedly accomplished, but, whether recorded or not by Mr. 
Marshall, I cannot sa 
I feel I am warranted in pronouncing the markings of the 
Navicula Spencerii shown as lines, not to have been a difficult 
test for the object glasses of the ‘‘ Londoners” at the time of its 
arrival. It appears it was so, however, for some observers, and 
Prof. Bailey drew from his correspondents’ letters the. very fair in- 
ference, that the instrument was at fault and not themselves ; he 
supports, ererer my conclusion, when he states that the micro- 
n the hands of their r possessors, had failed to show 
the ai itees. sod resolved them in his. 
With regard to the measurements of the distance of the marie 
ings, fixed by Prof. Bailey at values so widely different from my 
own, I have to remark, that I delayed answering his communica- 
tion, in order to repeat my measurements with everypossible pre- 
caution 
Prof. Bailey states the distances of the markings to be from 
rrvsasth tO g5y'sazth of an ineh, whilst I assign to them a 
value of from ;;4,;,;th to ae 4 of an inch, (from centre 
. — of the spots.) That the measurement given by me 
t very wide of the truth, I will proceed to show, and I 
Scliews that Prof. Bailey, on reference to his notes or on repeating 
the measurement, will trace out the cause of the error he has 
fallen into 
My previous measurements were made with a ruled microm- 
eter, used with a positive eye-piece, the value of whose divisions 
had been rigorously estimated by comparison with a micrometer 
of ;,';,th of an inch, placed on the stage of the microscope. 
have now repeated the estimation by a different, and I believe 
.  ageeptd method to that I employed before, viz., by illumina- 
ing the object to show alternately the markings as cross and 
icmeendion lines, and then drawing them, by the aid of a small 
toe reflector, shown at page 129 of Mr. ‘Quekett’s work. Re- 
ing the microscope in exactly the same position, I removed 
i object, and placed on the stage of the mi oe ig a mi- 
