Patt re? om 
Aug. 24, 1871 | 


SUGGESTIONS TO OBSERVERS OF THE 
SOLAR ECLIPSE OF DECEMBER NEXT 
28 analysis of the observations made at the recent 
eclipse seems to show that it would be desirable that 
three or four observers should be stationed some 10 miles 
i the north or south of the limits of totality to watch 
or— 
1. Shadow bands passing along the ground, and to note 
carefully the direction of their motion and their velocity. 
If possible this should be done upon any three planes at 
right angles to one another, say the ground, an east and 
west wall, and a north and south wall. 
2. Direction and velo.ity of the wind, and also the 
direction and velocity of the drift of the clouds. This 
should be done by observers associated with those 
employed on (1). 
3. Nays or brushes of light from the thin cusps of the 
sun, and whether they alter their position and intensity. 
4. Spurious red prominences from the thin crescent, 
noting carefully their position and size. 
The following instruments might be used with advan- | 
tage for polariscopic observations :— 
1. A polarimeter, consisting of a Savart’s. polariscope 
~mounted behind four plates of crown g'ass movable on 
an axis, perpendicular to the plane of polarisation, which 
is furnished with a graduated circle and pointer, to re- 
gister the position in which the depolarisation of the plates 
neutralises the polarisation of the object. This should be 
used with a telescope having a diaphragm in the common 
focus restricting the diameter of the field to about 10% 
The attention of the observer should be confined to the 
centre of the field (when the bands are made to disappear 
by rotating the plates). 
2. A plate of right and left-handed quartz attached in 
front of a posilive eye-piece, so as to lie in the common 
focus of the telescope and eye-piece, with an analysing 
Nicol placed between the lenses of the eye-piece, which 
should have a field about 1° in diameter. 
The bi-quartz should be fixed so that the line of junc- 
tion marks the plane of polarisation, giving the two halves 
purple alike. 
The following polariscopic observations are suggested : 
1. To determine the plane and measure the amount of 
atmospheric polarisation at at least three points, about 8° 
or 10° away from the sun’s place. The points to be chosen 
round the sun, say N,, N.E., and E., considering the sun 
asa map. (It would be well if three observers could 
be appointed each to take one fixed spot for atmospheric 
polarisation and to note the changes which take place 
during totality—both in the plane of atmospheric polari- 
sation and in its intensity.) 
2. The intensity of polarisation should be carefully 
measured with the polarimeter at different points of the 
corona, the observer taking care to notice when the | 
Savart’s bands disappear in the cev¢re of his field as before 
stated. 
3. The dark moon and corona should be bisected by the 
line of junction of the bi-quartz polariscope, and the colours 
upon the corona should be carefully noted, not only near 
the line of junction, but also round the whole circum- 
ference of the dark moon. Any sudden transition from 
one colour to another should be especially recorded. 
Should any ray, or rift, or sector of colour with sharp 
edges be observed, it would be well to place the line of 
junction across such sector or rift, and note the colours 
upon its edges ; the telescope carrying with it the line of 
junction might then be slowly withdrawn along the sector 
or rift from the limb of the moon outwards until all indica- 
tions of the rift or its edges are lost—the observer, of 
course, noting the plane of polarisation within the rift, 
and whether it differs from that of the air polarisation in 
the neighbourhood of the sun. 
A. C, RANYARD 
NATORE 


NOTES ON ECLIPSE PHOTOGRAPHY 
bos of the sun occurred in 1860, 1868, and 1869, 
when photography was employed chiefly to obtain 
evidence as to the nature of the red prominences, and in 
all cases a telescope of some kind was used, the image 
being taken at the principal focus. 
It is, however, preferable that instead of a telescope an 
ordinary photographic Jens of /ozg focus be employed. 
Such alens may be a portrait combination, or single or 
compound lenses, adapted for landscape or copying work ; 
the conditions are that the image shall be as large as 
possible, and the lens quick-acting, These requirements 
were found to be combined in the lens I used at Syracuse, 
and which was made by Mr. Dallmeyer, and lent to me 
for the purpose of the expedition. The lens is gin. in 
aperture, and has a focal length of 30in, the image of the 
sun or moon being three-tenths of an inch in diameter. 
It is scarcely to be expected that there will be many 
lenses of this exceptional class available in India; but 
no doubt, there are many good landscape lenses of long 
focus which may be used, and if the images they give 
are not so large as those taken by the 4-inch “rapid 
rectilinear,” the pictures obtained may have scientific 
value, although small. It is an interesting fact that at 
Oran Dr. Huggins had arranged with a local photographer 
to attempt some pictures with a small lens, giving an 
image of about 1th of an inch in diameter. 
In all cases it is strongly to be recommended that the 
best instruments be used. Instead of Dallmeyer’s 4 inch 


-PLARN- 
lens, it is suggested that Jenses of the “ rapid rectilinear” 
make, of still larger aperture, be used, as the focal length 
will be increased, and consequently a larger picture will 
be obtained, allowance, of course, being made for the in- 
creased exposure required, if the focal length of the lens 
has been made greater in proportion to the aperture. 
It will be convenient to assume in what I have now to 
say that a lens of at least four inches aperture and thirty 
inches focal length will in all cases be used, and that it 
it will be corrected for the chemical rays. 
Now an instrument of this kind, if used to photograph 
the sun’s corona, wiil be useless if not mounted so as to 
follow the sun’s apparent motion. The camera must, 
therefore, be mounted on a stand having clockwork 
motion. The stand of an equatorial telescope is what is 
required. The telescope may be removed, and the camera 
fixed in its Place—this for convenience only—or the 
camera may be fixed on the top of the telescope. It 
wiil, however, be better to remove the telescope. 
In the observatory and dark room as used at 
Syracuse, the framework was of wood, as slight as 
possible consistent with stability, and was covered 
entirely with waterproof cloth, the dark room being 
lined with yellow calico, in some parts double. The 
floor was also covered with the waterproof cloth to 
keep down the dust. Instead of the cloth for the sides 
and the roof I should prefer very thin boards, and the 
roof only need be made watertight, the edges of the wood 
being made, if necessary, to overlap. If wood be used, 
one thickness of yellow calico will be sufficient. The 
