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XX. Spectroscopic Observations of the Sun. 
By J. Norman Lockyer, F.B.S., and G. M. Seabroke, F.B.A.S. 
Beceived February 2 , — Bead March 19, 1874. 
We have the honour to communicate to the Eoyal Society the accompanying Spectro- 
scopic Observations of the Chromosphere and of the Sun generally, made during the 
period between the 1st October, 1872, and the 31st December, 1873. 
The London observations have been made in Alexandra Road, Finchley Road, N.W. ; 
the Rugby observations in the Temple Observatory at that place. 
The following details are given of the instruments and methods of observation 
employed. 
LONDON OBSEBYATIONS. 
A 6^-inch refracting telescope by Cooke, of York, mounted equatorially, was employed, 
to which is attached the 7-prism spectroscope by Browning, of London, already described. 
A position-circle, made by Cooke, of York, was used for obtaining the position-angle of 
the prominences and of the various details of the chromosphere. 
On the side towards the spectroscope the circle is provided with a pinion, which, 
acting on a circular rack, causes the graduated half of the circle to rotate, the vernier 
being on the fixed half attached to the telescope-body. 
On the 16th of September, 1873, the prisms spectroscope was replaced by a diffraction- 
grating of speculum-metal containing 6121 lines to the inch, made by Mr. L. M. Ruther- 
furd, of New York, by whom it was generously placed at Mr. Lockyer’s disposal ; 
the whole apparatus is only 15 inches in length, and weighs 3 lbs., while the 7-prism 
spectroscope, with its mounting, is 24 inches long, and weighs lOf lbs., the principal 
weight, moreover, being 18 inches from the end of the telescope. In dispersive power 
the 2nd order spectrum of the grating is equal to 7 prisms, while with equal dispersive 
power the grating gives much more light. 
The positions of the prominences have been determined as follows : — 
Standing with the back to the sun, and looking at the sun’s image on the slit plate, 
the bottom of the image, being the image of the real North of the sun, is called North, 
the left-hand side of the image East, the right hand West, and the top South. The 
degrees are reckoned from North as zero through East to North again in the same 
direction as the hands of a watch, N., E., S., W. of the image on the plate being of 
course in the contrary direction to N., E., S., W. as seen directly on the sun. If, 
MDCCCLXXV. 4 H 
