SPECTKOSOOPIC OBSERVATIONS OE THE SUN. 
579 
limb of the sun’s image to traverse the disk, and then turning the instrument round 
until the limb exactly passes from one wire to the opposite one ; then, on bringing the 
sun’s image concentric with the disk, the left-hand wire, as seen by looking on the 
disk with the back to the sun, corresponds to the East side of the sun as looked at 
directly, and is therefore at the position of 90°, and the right-hand one corresponds 
to the W. of the sun 270°; the lowest wire will then correspond to the North or 0°, 
and the upper to the South or 180°. The direction of reckoning the degrees is as usual 
N., E., S., W., or as looking directly at the sun in the contrary direction to the hands of 
a clock ; but as looking on the disk with the back to the sun N., E., S., W. will be in 
the same way as the hands of a clock ; and if the ring of chromosphere, as it would 
appear to an observer looking at it in the annulus with the back to the sun, be cut at 
N. or 0° and straightened, the appearance would be that shown in the drawings ; although 
the annulus of chromosphere is looked at with the spectroscope from the opposite side 
to that of the sun, the image is half inverted by a diagonal reflecting prism in the 
telescope of the spectroscope, so that its appearance is the same as if looked at in the 
annulus from the same side as the sun. The width of the annulus through which the 
light from the chromosphere passes is such that a prominence 100" in height reaches 
across the annulus, so that the height of the prominences can be judged of with fair 
accuracy. 
We have purposely refrained from any reduction of these observations, as we are of 
opinion that such reduction will be most usefully made when the observations of the 
Italian and other observers have been published, as it is hoped that the English and 
foreign observations may be in some cases so complementary of each other that long 
gaps may be avoided. 
Notes to accompany the Maps. (Plates 59 to 64.) 
(LONDON OBSERVATIONS*.) 
December 6th, 1872. — Chromosphere generally 10" high. 
January 1st, 1873. — Chromosphere about the usual height, except 150°-170°, where 
it was low. 
March 8th, 1873. — Chromosphere very hair-like in its outline, about 12" high. 
Between 3.30 and 4.0 p.m. a large spot was observed between 240° and 250°, and close 
to the limb. Violent action was going on. C was intensely black over the spot, and, I 
think, slightly thickened ; D was very thick, and bent towards the red. The magne- 
sium lines did not seem to be affected; but the two lines of b, 5166'5-f and 
( Fe 
* With these are included, in order of date, nine woodcuts of the more remarkable prominences, of the size 
of the original drawings, which were made, some at London by Mr. Eriswell, some at Rugby by Mr. Seabroke. 
The locality, date, and position-angle, which are given in each case, will enable the reader to find the places of 
these prominences in the Maps. 
4 h 2 
