460 
CAPTAIN' W. DE W. ABNEY ON THE SOLAR SPECTRUM. 
its bearings with a slow motion. In reference to this I may mention that nearly all 
friction between the brass tube and the wood is avoided by placing between them 
pieces of baize or cloth. With a concave grating the necessity of the slit being 
accurately parallel to the lines of the ruling cannot be too much insisted upon, since 
the definition very largely depends upon it. 
Fig. 2. 
The majority of the photographs which have been measured were taken with the 
flat grating, and as the apparatus differs slightly in its details from that previously 
described, I annex a drawing showing how it was employed. 
S is the slit; a beam of light falls on the right-angle prism p, which reflects it to 
the mirror M', whence it by successive reflections from G and M" falls on P, the 
photographic plate. The mirrors M' and M" have radii of curvature of about 7 feet 
each ; M' is so placed that the rays from S are made parallel and fall on the 
grating G. The spectra thus formed are very bright and the definition excellent, and 
I can strongly recommend this arrangement for spectrum work, whether gratings or 
prisms are employed. The object in using this arrangement was to secure the focus 
of all overlapping rays being in the same plane, thus allowing coincidences between 
the first and second and the second and third orders to be discussed. 
For filling the gratings with light a quartz or glass condensing lens was employed, 
though a silvered mirror was also utilised. The sunlight was reflected from a helio- 
stat, the mirror being silvered on the front surface. In reference to this I may say 
that Colonel Festing and myself have examined the reflective power of silver and a 
