286 PROFESSOR PIAZZI SMYTH ON 
tures to prisms, and lenses, from 1:0 to 2°25 inches wide; the iength of the 
collimator being 31 inches ; and that of the telescope rather more. 
The Sun’s light was brought to the instrument by a heliostat mirror and lens 
of long focus, worked by endless screws and managed very steadily for me 
during the whole of the observations in both years by Mrs Piazzi SmyTu. 
Generally speaking too, all the apparatus, though its fittings were rough, 
answered well within its limits of power; excepting only this anomaly, 
that between E and F, from some cause I have not yet been able to 
ascertain, the telescope would not focus accurately ; or rather it had two 
foci thereabouts, and neither of them sharp, no matter how fine the slit was 
made, or how carefully the prisms were re-adjusted to position of Minimum 
Deviation for precisely the part of the spectrum there concerned ; while a very 
fine slit, when out of focus, makes some puzzling lines of its own; and may 
have occasionally increased unduly the number of the thinner ones noted for 
the Sun. 
Again after passing G, and more especially after passing little h, the con- 
tinuous spectrum’s light was too faint for good observation. This is indeed a 
region to be recorded by photography rather than the eye; and it is being so 
tabulated in a magnificent manner by Mr RurHerrurp (New York), Mr Lockyer 
(London), Dr Henry Draper (New York), M. Cornu (Paris), and many other 
most able savants. So that for full accuracy, number of lines, and extent of — 
spectral range into the ultra-violet, and more especially the fluorescent regions, 
their works should of course be referred to. But without presuming to com- 
pare with them in any degree, I trust there was no objection to my recording 
for the violet, as I had done already for the red, end of the spectrum, exactly 
where both lines, and continuous spectrum light ceased to be visible to the eye; 
especially as I made rather a point of abolishing the use of coloured glasses, so 
generally used by other observers, to prevent false glare in the field of view, by 
employing more or less of preliminary prism-separating power instead, with 
several unexceptionable advantages. 
All this however has merely to do with the ways and means, which may be 
various with different scientists, towards obtaining one and the same end; viz., 
procuring a continuous record of an eye-observed and glass-transmitted solar 
spectrum ; with, if possible, some improvement or extension over anything of 
the same kind yet made public. Now the documents to be competed with are 
not very numerous. In fact, the only published Solar spectrum that has any 
claim towards being both large, accurate, complete, and registered on an absolute 
or universal scale, is the map by the late admirable Professor ANGstrom, of Upsala, 
worthily called his “ Normal Solar Spectrum,” and referred to now by savants 
of all countries. It is about 137 inches long, and contains the places and 
physiognomies of about 1400 lines. 
