LYMAN. — THE SPECTRUM OF IIYDKOGEN. 14.') 
movable stand and the grating spectrum was projected upon it by means of a lens. 
By changing the magnification so as to keep step with the dispersion the strong lines 
on the one spectrum were identified with those in the other from X 1074 to X1269 
without the least difficulty. From the values thus obtained interpolation curves were 
drawn for each one of the twelve plates separately and by means of these curves a 
scale of Angstrom units was attached to each of the twelve illustrations. By the per- 
mission of Dr. Schumann and through the kindness of Professor Langley of the Smith- 
sonian Institution the half-tone reproductions which appear at the end of this memoir 
were then made from these illustrations. They by no means do justice to the fine 
originals, but considering the difficulty of the process they may be considered fairly 
satisfactory. 
The agreement between the author's measured values and the prismatic lines is 
extremely gratifying. Of the two hundred and eighty-five lines given in the tobies 
all but three or four are found in Schumann's plates. There are, however, a consider- 
able number of fainter lines in the prismatic spectrum not visible in the plate 
obtained with the grating. Moreover, owing to the fineness of the slit, and the great 
dispersion used by Schumann some of the single lines of the table are seen by compari- 
son with the prismatic spectrum, to consist of doublets 
The excellent agreement betw 
pectra obtained under such differ 
conditions makes the existence of any chance impurity very improbable. 
The extreme line in Schumann's map has the value X 1266.9. That author has 
stated that he obtained some lines too faint to reproduce ; from the angles given ■ it is 
difficult to calculate their exact wave-length, but it seems improbable that they should 
have a value much below X 1230. In this connection it is interesting to note that the 
calculation of Martins • from the Kettler-Helmholtz formula for fluonte was not far 
wrong. 
LIMIT OF THE SPECTRUM. 
It may well be asked, - to what is the present limit of the spectrum due ? There 
ar? several causes which go to make up an answer to this question. 
A much longer exposure might result in the discovery of new lines; unluckily 
difficulties in the way of this seemingly simple step. For, as has been pre 
viously stated, with a windowless tube there is a great tendency for the discharge to 
spread into the receiver and cause fatal fogging of the plate. No plan has so far been 
devised to obviate this difficulty and up to the present the length of exposure has 
• Smithsonian Contributions, 1415, p. H. 
I F. P. Martins Ann. der Physik, W01. Heft IL, p. BIS. 
