570 
MR. J. NORMAN LOCKYER ON SPECTRUM-ANALYSIS 
of photography, and although very incomplete so far as regards metallic coincidences, 
it has proved of invaluable service in the construction of the present map. 
That the value of the photographic method has not been over-estimated will be 
rendered evident from the following considerations. 
In the first place, it is well known that a great extent of the spectrum totally 
invisible under ordinary circumstances can be recorded by means of photography. 
Thus, taking the length of spectrum which can be conveniently mapped by eye to 
extend from h (wave-length 4100) to near A (wave-length 7310), this map would be, 
on Angstrom’s scale, 3‘20 metres long. Taking about Q (wave-length 3177, Mas- 
cart) as the most refrangible limit of the spectrum conveniently workable by photo¬ 
graphy, we thus gain from this point to In, on the same scale, an additional length of 
nearly 1 metre. 
Next with regard to the amount of detail obtained from photographs compared 
with that observable by eye. To illustrate the advantage of the present method, 
it will be instructive to compare a given region mapped by eye and by photography. 
The portion first selected has been mapped by Angstrom, by Cornu, and likewise in 
the course of the present work. 
Solar Spectrum, Section 4000-4100. 
Eye observation. 
No. of 
lines. 
Photographic record. 
No. of 
lines. 
“ Spectre Normal” .... 
32 
Cornu’s map. 
Map constructed from Ruther- 
104 
furd’s print of the solar 
spectrum. 
297 
Map constructed from nega- 
tires taken July, 1875 (inclu¬ 
ding lines glimpsed merely) 
333 
In a region more easily visible than the above, the same superiority is exhibited by 
the photographic spectrum. 
Solar Spectrum, Section 4200-4300. 
Eye observation. 
No. of 
lines. 
Photographic record. 
No. of 
lines. 
“ Spectre Normal ” .... 
94 
Map constructed from Ruther- 
furd’s print of the solar 
spectrum. 
Map constructed from a nega¬ 
tive taken July 29, 1875 
275 
412 
Seeing the great advantages thus offered by photography, even in a part of the 
spectrum distinctly visible, I am persuaded that it will be necessary to re-map the 
