H. Draper—Diffraction Spectrum Photography. 403 
3440) of about 12 inches (‘305 meter) long. I have succeeded, 
however, in photographing from near b (wave-length 5167) to 
T (wave-length 3032) by resorting to a ruled speculum plane 
and a concave speculum mirror, but the photographic and opti- 
cal difficulties in securing an enlarged spectrum of that length 
are great. 
Of course, in such a research as this an essential is a finely 
and evenly ruled plane of glass or other material. Those which 
Ihave used were made by a machine devised and constructed 
1843, except that I have not silvered the ruling, and therefore 
have used the refracted, and not the reflected beam. The slit 
order, which has certain conspicuous advantages. In the first 
Place, it is dilated to such an extent as to give a long image, 
*pectrum. It has been commonly supposed, until the recent 
memoirs of J. W. Draper, that there are in the spectrum three 
*Since writin, : hing the lines of the 
isi g the above I have succeeded in photographing the : 
inch g,SPectrum from 6 downward, and the picture comp sr 
y, below’ =D; O, B, a and A, but also the ultra-red rays. The great groups 4 P, 
1 tlow A, discovered by my father in 1843, are distinctly reprodu 
