W. Dela Rue on the Navicula Spencerii. 25 
The object alluded to, was a slide of the N. Spencerii mount- 
ed in balsam, and sent over to Mr. Marshall by Prof. Bailey ; two 
rings marked with a diamond, indicated two objects to be exam- 
ined. The test having been illuminated with oblique light, from 
a lamp with and without the use of the mirror, was examined 
with a Ross’s ;;th having only an aperture of 90°; this ebject- 
ive had been in m possession for some years, and was far infe- 
rior to those Ross was furnishing, with an aperture from 115° 
* 
Very little difficulty was now experienced in bringing out sep- 
arately both sets of lines most distinctly, by illuminating in two 
different directions. We now attempted to examine the object 
with a Ross’s ,';th, of 110°; but, owing to the thickness of the 
4 coveriug-glass, we could not get it on the object ; as it was an 
; only 2 gp we did not deem it advisable to change the cover- 
ing-glass 
In consequence, Mr. Marshall undertook to write to Prof. Bailey, 
to request the favor of a little of the deposit, in order to mount 
t fresh specimens, both dry and in balsam; the former with the 
| view of studying the nature of the markings under the most fav- 
orable circumstances. 
I believe Iam right in saying that the first parcel which arrived 
was lost by an accident; at all events, it was not until the 14th 
of August. 1848, that Mr. Marshall had prepared specimens with 
covering-glass sufficiently thin for our large aperture ;';ths. — 
that day he wrote to me, “I have prepared two sides for you, 
one in balsam and the other dry, they have,been both evaporated 
on bi covering-glass which is exceedingly thin. * * * * I should 
to go over the slides 1 have prepared, and particularly the 
ios mounted one, for the purpose of resolving it, if we can, by 
direct light. The Bank is so disturbed by the heavy omnibuses 
that your own quiet room is the best place, and if you should be 
disengaged some evening, I will go to your house, taking with 
me se" slides and two or three others which I have for your 
cabine 
Reva to his Felting the above, we had succeeded in 
owing the markings with a ~ aperture 4th inch objective of 
Ross's make, having an aperture of 80°. This fact was com- 
municated verbally to the vpeehinc 4 Society, on one of the 
ordinary meetings, by Mr. 
aving again met at iy eae we examined the dry and 
Isamed specimens; as we were well acquainted with the 
N. angulata, we at once proceeded with the view of ascertaining 
= s Si ae St 
a 
* 
* Mr. Ross, some time back, succeeded in making a coset with an cache of 
_ 1359, in which the aberratio ns were well corrected. It is now in the possession of 
Dr. Leeson, 
Szconp Sznms, Vol, IX, No. 25.—Jan,, 1850. 4 
