lie ON A TABLE OF STANDAED WAVE LENGTHS OF THE SPECTEAL • LINES 
Thus the region 3900 in the solar spectrum has been obtained from both 
wave Ieii"ths 5200 and 5850. The mean of these gave values of the substandards 
for working up the plates taken at this pointy and containing also metallic lines 
at 2700. 
A<'ain. the boron lines at 2496 and 2497 have been obtained from the reo-ions 
4S00, 3200, and 3600. The mean values give substandards for working up the 
■ 
luetuIHc spectra of that region. Also the near coincidence in the values of the 
wave lengths of these lines Indicate the relative accuracy of the regions 2496, 3200, 
oGOO, and 4800. 
The use of these substandards is as follows. The photographic plates, mostly 
]0 inches long, were measured mostly on a machine giving wave lengths direct. 
The difference of the results from the substandards were then plotted on a paper 
the curve of correction for length upon it in such a way that the final marks 
should theoretically be a straight line. This was actually the case in all but a few 
plates, in which the cnmera was displaced. A straight line was then passed through 
all the marks as nearly as may be, and the correction taken off. This correction 
could thus be obtained to jj\-^ division of Angstrom, and amounted to only a few 
hundredths of a division at most. Possibly j\ division of Angstrom was the greatest 
o 
rection required for len 
In this way each plate 
of all the wave length d 
tions throughout its extent, and will not admit of any correction save a linear one 
should such ever be required in working over the table again. 
In every plate having a solar and metallic spectrum upon it, there is ofte 
indeed always— a slight displacement. This is due either to some slight displace- 
ment of the apparatus in changing from one spectrum to the other, or to the flict 
that the solar and the electric li,srht pass throudi the slit and fall on the grating 
Q .- ^ ^^^^^ uiAi yj 11 i^ 
difierently. In all cases an attempt was made to eliminate it by exposing on 
the solar spectrum, both before and after the arc, but there still remained a dis- 
placement of jI^ to ^fo division of Angstrom, which was determined and cor- 
rected for by measuring the diiference between the metallic and coinciding solar 
hues, selecting a great number of them. 
f possibl 
The changes from sun to arc light are much more extensive than from one order 
of solar spectrum to another. In two cases T have tested the latter, and found no 
displacement, and have no fear that it exists in the others. 
In workmg up the plates, I have started at the plates whose centre is at wave 
length 4600, and proceeded either way from that point. For this purpose I have 
