- 
28 W. Dela Rue on the Navicula Spencerit. 
pense ; these have some of the finer series crossed at angles 
and 120° respectively, and afford a good control of the 
i a one’s conclusions respecting the nature of their ——— 
All the straight bands, with the exception of the last, can 
made out by “oblique light with my quarter of 80° before slluded 
to. ‘The last series of Yeealve of an inch I can see to be lined, 
on using a ;;th object glass of 110°, but up to the present mo- 
ment, I have not satisfied myself that the lines I see do not com- 
prise two of the real lines. It is, at all events, worthy of experi- 
mental inquiry, to ascertain whether the physical properties of 
light donot put a natural limit to our resolving lines so close as 
the ;5;';;;th and the ;,,'s;;th of an inch, quoted by Prof. mers 
the celebrated Fraunhofer maintained that this was the case 
After what I have said, in the commencement of this commu- 
nication, it is hardly necessary for me to state that I quite agree 
with Prof. Bailey, that the markings on the Spencerii are due to the 
allinement of prominences; I moreover concur in his views res- 
pecting the difficulty of deciding on the real nature of the mark- 
ings, on objects so minnte as ‘the Naviculacee. One thing is 
owever quite certain, that a much inferior glass, provided it have 
a sufficient angle of aperture, will suffice t o show even both sets 
of lines at one time, than that required to ee them out as dis- 
tinct dots without any blue or fuzziness. Bringing out both sets 
of lines at once is to me a well know phenomenon and quite dif- 
ferent from the exhibition of markings by the image of a lumin- 
ous object brought to focus in the plane of the object. 
I must not be understood to affirm positively that Mr. Spencer’s 
glasses will not do this, for I have never had an opportunity of 
examining one. On a late occasion a recent objective of Spen- 
cer’s was however tried in my presence by the possessor on the 
N. angulata, but, though very excellent, it did not equal our English 
twelfths. Judging from Mr. Spencer's. roduction, I feel assured 
that he is a man of too much merit to feel hurt at this criticism. 
As [am unacquainted with Mr. a. new test, I cannot 
speak as to its difficulry. 
would recommend* Natchet’s condensing prism to the atten- 
tion of all microscopists engaged in the examination of lined ob- 
jects; it brings out the dot-like markings of the N. Spencerii with 
remarkable force, even in balsamed specimens. It consists of a 
prism, which, by two internal reflections and the inclined convex 
surface of its summit, condenses light at an angle of 35° on to 
the object. By mounting it in such a way that it may be tee 
Cae as brought to a focus, it answers all the purpose 
g’s stage. 
* Mr. Natchet is an optician resident in Paris 
