- 
NATURE 
77 
COMPARISON OF THE SPECTRA. OF THE 
LIMB AND OF THE CENTRE OF THE SUN* 
if: COMPARISON of the spectrum of the edge of the sun 
with that of its centre is of great theoretical interest; but 
any comparison other than by direct juxtaposition must be very 
unsatisfactory, and the more so as the differences are less. In 
order to obtain spectra of two different portions of the sun side 
by side, where the slightest variations may be detected, I have 
constructed a small prism with four polished sides, its bases 
being parallelograms. This is so placed that one face rests upon 
the slit plate of the telespectroscope, and has its acute edge per- 
pendicular to the slit at its middle point, The instrument may 
then be directed so that the image of the sun falls with its centre 
on the uncovered portion of the slit, while the light which forms 
the edge of the sun, falling perpendicularly upon the first sur- 
face of the prism, suffers two interior total reflections and a dis- 
placement depending upon the form of the prism. A glance at 
the figure, in which ss’ is the slit, L L’ the diameter of the sun’s 
image, and P the prism, shows that no light from the covered 
of the slit will reach the collimating lens except that which 
been reflected from the two sides of the prism. The relation 
of the acute angle (v) and the distance between the reflecting 
sides (¢) to the focal length of the great telescope (7) and the 
width of the spectrum (a) is given by the formula, 
2¢ sin v = F tan 16' — a. 
The sides of the prism not fixed by the equation admit of con- 
siderable latitude, but should be made to approach the lower 
limit in order that the planes of the direct and transmitted 
images may be as little separated as possible. Of course ¢ 
saad should be so proportioned that the reflections may be 
total. 
The instruments with which the following observations have 
been made are those belonging to the observatory of the Sheffield 
(U.S.) Scientific School, consisting of an equatorial telescope of 
gin. aperture, and 118 in. focal length, by Clark, and a spectro- 
scope of Young’s form by the same maker. The spectroscope 
has a dispersive power of 12 prisms of 60°. In most of these 
observations an eye-piece of high power has been adapted to it, 
which gives a separation of the D lines equal to 64 minutes 
nearly. In the small prism placed before the slit, @ is equal to 
*o4in., a quarter of the length of the slit. 
When the instrument is properly directed and in adjustment, 
we see a very narrow black line dividing the spectrum longitu- 
dinally into two parts of widely different intensity ; the fainter, 
belonging to the limb of the sun, is marked on its edge by 
the bright chromosphere lines. Upon comparing these two 
spectra, certain differences are recognised besides that of in- 
tensity, by far the most marked of which are exhibited by the 
lines 4, and 4,, which become sharper and less hazy near the 
limb. The line 4, possesses the same characteristic, but toa 
less degree ; C and F also become sharper in the same region. 
Excepting these and the D lines it requires very close examina- 
tion to detect any variation. There is, however, a line in the 
red at 768°: of Kirchhoff’s scale which is strongly marked near 
- the centre of the sun’s disc, but disappears entirely, to my 
wer at least, within 16’ to 20" from the limb. Two other 
ines below F, at 1828°6 and 1830°9 of the same scale, exhibit 
nearly complementary phenomena, #.c., they are strongly marked 
near the edge, but much fainter at the centre. These latter lines 
also become greatly strengthened over the penumbrz of spots. 
The line 768°1 is not thus affected. These areall the differences 
which I have invariably seen in repeated examinations since 
February 17. 
Others have, however, been suspected. Certain lines, which 
are strengthened in a region of spots like those above men- 
tioned, appear to be strengthened also near the edge, but do not 
* Made at the Sheffield (U.S.) Scientific School, Communicated by 
Prof. Newton. 
undergo so marked a change. It is obvious that the differences 
should be most pronounced in the clearest sky, and such is the 
case. The closest examination has extended only from B to a 
short distance above F, as the plate glass of which the small 
prism is made has a decided yellow tint and absorbs the blue 
rays strongly. 
Since the light from the border of the sun undergoes a gene- 
ral absorption, which reduces its intensity to much less than 
one-fourth that at the centre, according to Secchi’s measure- 
ments, and yet the spectroscopic character is changed so slightly, 
it is impossible for me to escape the conviction that the seat of 
the selective absorption, which produces the Fraunhofer lines, 
is below the envelope which exerts the general absorption. But 
the phenomena of the faculze prove not only that this envelope 
rests upon the photosphere, but also that it is very thin, The 
origin of the Fraunhofer lines, then, must be in the photosphere 
itself, which is in accordance with Lockyer’s views. 
Any effects which the chromosphere might produce, we would 
anticipate finding most evident in the lines of those gases which 
are readily detected there. A reference to the observations 
shows at once a compliance with this anticipation in the lines of 
hydrogen, magnesium, and sodium. The line 768-1 is not less 
strikingly in concordance, if it be regarded as 768-?* (the ? in- 
dicates doubt as to the tenths of the scale, and * absence of a 
corresponding black line) of Young’s Catalogue of Chromo- 
sphere Lines. The lines 1828°6 and 1830°9, with others of the 
same class, probably have their origin in the medium which ex- 
erts the general absorption, and thus are allied to our telluric 
lines. It also seems probable that the chromosphere is too 
transparent to reverse many of its lines. That this is the case 
in the helium lines is tolerably certain. 
In the apparatus described, two similar prisms were also placed 
over the slit in a symmetrical position. The spectra of two 
opposite edges of the sun were thus brought together, and the 
change in refrangibility due to the sun’s rotation was very clearly 
shown. Cuas H. Hastincs 
Newhaven, April 3 
THE “INSTINCT” QUESTION 
pew the many additional communications we have 
received on this subject, we make the following 
selection. 
With regard to a sense of direction, Mr. George C. 
Merrill, of Topeka, Kansas, writes as follows :— 
I have learned from the hunters and guides who spend their 
lives on the plains and mountains west of us, that no matter 
‘| how far or with what turns they may have been led in chasing 
the bison or other game, they on their return to camp always 
take a straight line. In explanation they say that unconsciously 
to themselves they have kept all the turns in their mind. 
Mr. C. Bygrave Wharton, of Bushey, Herts, writes :— 
As a left-handed and left-legged man*who has more than once 
been lost in the bush in New South Wales, my experience may 
possibly be of interest to Mr. George Darwin and others. In- 
variably I unintentionally bore to the left ; and once, after wan- 
dering for about six hours, just as I was giving myself up for 
lost, I discovered that I was within a hundred yards of the place 
from which I had started having performed a large circle to the 
left. It will thus be seen that though my left leg and arm are the 
aap there is always a tendency to walk in a circle to the 
left. 
Mr. William Earley, of the Gardens, Valentines, sends 
the following interesting observations on the habits of 
wild rabbits :— 
As is well known, the doe rabbit does not produce her young 
in any ordinary rabbit warren, or ‘‘run,” but invariably selects 
a quiet out-of-the-way situation wherein to form a nursery for 
them. Now the reason for this peculiar practice has always 
been attributed to the fact that they leave their legitimate homes 
at this all-important period, simply because the male parents 
invariably destroy the offspring if an attempt be made to breed 
them in the permanent home or warren. I incline to believe we 
must look elsewhere for the explanation. 
Firstly, then, a close atmosphere seems all-important to their 
development, as the old doe rabbit not alone denudes her breast 
of its natural fur covering wherein to ensconce them warmly all 
