May 13, 1886] 
NAL RE 
AS 
co}2) 
is single, and leads downwards and backwards in the 
median line, being enveloped in the tissue passing from 
the foramen directly to the roof of the thalamencephalon. 
I have not yet actually traced the nerve itself into the 
brain, but it is difficult to imagine that it can possibly 
arise as a branch from a cranial nerve, being szzg/e and 
medianly placed, and, as just said, enveloped in the 
material running directly to the roof of the brain from the 
foramen. 
There can be little doubt that it represents the stalk 
connecting the distal with the proximal outgrowth from 
the roof of the thalamencephalon, this part having ap- 
parently disappeared in other reptiles and amphibia (so 
far as is yet known). 
This being the case it is extremely interesting to ob- 
serve that another instance will be added to that of the 
optic nerves in which an, at first, hollow outgrowth from 
the brain becomes solid and transformed into a nerve, 
and further that the latter, as in the former case, is con- 
nected with an organ of vision. 
Though it is difficult to imagine what can be the use 
of the organ in its present state, seeing it is deeply em- 
bedded in connective tissue—so deeply as almost to 
Fic. 2.—Transverse section through parietal foramen of Ha tera punctata. 
The part below the dotted line is situated within the parietal foramen. 
preclude the idea of its being affected by light—yet it is 
important as showing in the same animal eyes developed 
in both the vertebrate and invertebrate type as regards 
the arrangement of the layers. 
In connection with this subject, it is perhaps of interest 
to point out that in formation of the paired eyes in- 
vagination to form an optic cup takes place, whilst 
apparently it does not do so in the case of what may be 
called the farzetal eye. A little consideration shows that 
the relative position of the rods depends entirely upon 
this invagination. In both cases they are formed upon 
the inner surface of the wall in the position correspond- 
ing to the epithelium of the neural canal: but in the one 
‘instance they are, by decay of the outer wall of the 
| invaginated cup, placed apparently on the outside of the 
optic vesicle ; whilst in the other instance they are 
formed in a similar position, but, as no invagination takes 
‘place and subsequent decay of one wall, they line the 
cavity of the vesicle. According to this we must suppose 
that the part of the wall where the lens is present has 
either disappeared or become modified into this. 
_ We may further observe that in both types of eye the 
‘nerve enters into connection with the elements on the 
f 
i 
_ 
| back to the vicinity of the meridian. 
| graphy. 
surface opposite to that on which the rods are developed. 
In conclusion my thanks are due to Prof. Moseley for 
his kindness in drawing my attention to the subject, and 
to Mr. E. B. Poulton, of Keble College, Oxford, who 
kindly placed two specimens of Hatteria at my disposal, 
and to Mr. Beddard, of the Zoological Society, for the use 
of another specimen. W. BALDWIN SPENCER 
Anatomical Department, University Museum, Oxford 
NOTE.—Since writing the above I have found the eye 
present in several other lizards, notably in Iguana, 
Chameleo vulgaris and Lacerta ocellata, and have traced 
the nerve into the proximal part of the epiphysis. 
ASTRONOMICAL PHOTOGRAPHY ' 
OME attempts made last year at photographing the 
heavens by means of an instrument quite rudi- 
mentary having yielded good results, the director of the 
Paris Observatory gave orders for the construction of a 
special apparatus, the design of which is shown in the 
accompanying figure (Fig. 1). The mechanical part has 
been executed in a highly remarkable manner by our 
accomplished artist, M. Gautier; the objective is our 
own production. 
This new instrument is composed of two telescopes in 
juxtaposition inclosed in a single metallic tube in the 
| form of a parallelopiped, and separated from each other 
along their whole length by a narrow partition. 
One of the object-glasses, with an aperture of 024m. 
and a focal length of 36o0m., is intended for eye obser- 
vation, and serves as a pointer. The other, with an 
aperture of 0°34 m. and a focus of 3°43 m., is achromatised 
for the chemical rays, and serves the purpose of photo- 
The optical axes of these two objectives being 
parallel, every star kept in the centre of the field of the 
eye-piece belonging to the first telescope produces its 
impression in the centre of the sensitive plate of the 
photographic apparatus. 
The equatorial is mounted in the form called English, 
| that is to say, the centre of the tube rests always in the 
polar axis of the instrument. This arrangement allows 
| of a star being followed from its rising to its setting with- 
out involving the necessity of bringing the instrument 
Like a common 
| equatorial it is furnished with hour circle and circle of 
| declination, and with a clock movement keeping the 
, mounting. 
apparatus in operation for three hours without fresh re- 
There are, moreover, independent very slow 
movements, whereby the axis of the telescope can be 
kept on a fixed point in the heavens, notwithstanding 
some slight irregularity in the movement of the clock- 
work, the orientation of the telescope, or the variations of 
atmospheric refraction. 
The photographic objective—the largest ever yet pro- 
duced—is formed according to a simple achromatic 
system, and, though of an extremely short focal length, 
is able, without the use of any diaphragm, exactly to cover 
the very considerable field of 3° diameter. 
Although but very recently mounted, this apparatus has 
already availed for the performance of numerous tasks. 
On star photographs it is possible to distinguish traces of 
stars of the 15th magnitude, too feebly marked, however, to 
bear transference on paper. The stars of the 14th magnitude 
are reproduced with a diameter of 1/40 of a millimetre. 
It is obvious that such small points might be liable to 
be confounded with the impurities of the sensitive coat- 
ing if the precaution is not taken to multiply the sta- 
tionary points. Each star is formed by a group of three 
points constituting an equilateral triangle, each side of 
which is no more than 1/12 of a millimetre. To the 
naked eye these three points appear to merge into one, 
but on eximining them with the aid of a somewhat 
t From an article by the Brothers Henry in La Nature. 
