Dr. H. Draper on Diffraction- Spectrum Photography. 419 



from below G (wave-length 4307) to above (wave-length 3440), 

 of about 12 inches ("305 metre long). I have succeeded, how- 

 ever, 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 optical 

 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 I 

 have used were made by a machine devised and constructed by 

 Mr. L. M. Rutherford, whose beautiful lunar and prismatic 

 spectrum-photographs are so well known to the scientific world. 

 The plate generally employed is of glass ruled with 6481 lines 

 to the inch; the ruled part is l-joo" * nc ^ ('027 metre) long and 

 Y(fe ('016 metre) wide. It is unquestionably much more nearly 

 perfect than similar gratings made by Nobert and others; for 

 the character of the photographs and the uniformity of the 

 orders on either side of the normal, together with its behaviour 

 under a searching examination, show that it leaves little to be 

 desired. As it is on glass and gives a bright transmitted spec- 

 trum, I have constructed the remainder of the optical apparatus 

 of glass achromatized according to the plan used by J. W. Draper 

 in 1843, except that I have not silvered the ruling, and there- 

 fore have used the refracted and not the reflected beam. The 

 slit is fy of an inch (*02 metre) long and y^j of an inch (-00023 

 metre) wide ; the jaws are of steel ; and there is not only a micro- 

 meter-screw for separating them, but also one for setting them 

 at an angle. Occasionally I have taken photographs with the 

 jaws -L of an inch ('00028 metre) apart at the top, and — L 

 ('00019 metre) at the bottom, so as to obtain different intensity 

 in the two edges of the spectrum. 



Most of the photographs have been of the spectrum of the 

 third order, which has certain conspicuous advantages. In the 

 first place it is dilated to such an extent as to give a long image 

 and yet one not too faint to be copied by a reasonable exposure 

 of the sensitive plate ; and in the second place the spectrum of 

 the second order overlaps it in such a way that D falls nearly 

 upon H, and b upon O. These coincidences are serviceable in 

 determining the true wave-lengths of all the rays. 



The only point of special interest in connexion with the pho- 



* Since writing the above I have succeeded in photographing the lines 

 of the visible spectrum from b downward ; and the picture comprises not 

 only the regions including E, D, C, B, a, and A, but also the ultra-red 

 rays. The great groups «, /3, y below A, discovered by my father in 1843, 

 are distinctly reproduced. 



2F2 



