W. Dela Rue on the Navicula Spencerii. 27 
crometer of ;;',;th of an inch which I sketched on the paper ; 
having by comparison roughly estimated the value of the mark- 
ings, [ then selected two sets of lines from Nobert’s test. slide, 
The drawing of the markings, that of the micrometer of 
sa'ssth of an inch, and those of Nobert’s series, were compared 
together by taking a sufficient number of lines in a pair of di- 
viders, and the distance of the markings thus ascertained. This 
Operation was repeated, first with an eye-piece, magnifying 1200 
diameters, then with one magnifying 1900 diameters. The dis- 
tances of the cross markings from two different sides of the shell, 
estimated with the first eye-piece, was ;;!,,th and ;z!,,th res- 
pectively, with the second eye-piece, the distances for the same 
part of the shell, were ;<},;th and ;;3,,th respectively. The 
longitudinal markings in both cases I found to be ;,4,;,th of an 
inch. All these values are from centre to centre, and are suffi- 
ciently concordant, amongst themselves and with those previously 
given, to establish my measurements beyond doubt. The differ- 
ence in distance between the cross and longitudinal lines, accounts 
for the greater difficulty one experiences in resolving the latter. 
Measurements of the markings of the hippocampus gave the 
following values: for the distance of the centres of the cross 
markings sath of an inch and for the longitudinal ;534,th 
of an inch; these were fully confirmed by comparison with 
Nobert’s lines of ;3/7;th and ,,1;;thof an inch, 
In order that others may be able to judge of the reliance to be 
placed on the values here given, I snbjoin the following measure- 
ments of some of Nobert’s lines, and the real values assigned by 
esc ; these were made without knowing at the time Nobert’s 
values: ° 
73 
ch; 
Lines to the inch (English). 
De la Rue Nobert. De la Rue Nobert. 
eaeel 11261 32000 32175 
13043 13056 37037 37537 
15200 15426 408 16 40950 
17647 18163 43103 42982 
20454 20475 45045 45016 
23666 | 23461 47468 47619 
27272 28153 50000 
I would wish here to record my admiration of the skill of Mr. 
Nobert, who has ruled at my request a series of progressive lines, 
fifteen in number, running from 11260 lines to 56300 lines to an 
inch, and a separate band on the same slide of 112613 lines to the 
inch, besides other series very useful to me in comparing with 
ie cee 
