88 
PROFESSOR W. N. HARTLEY AND MR. W. E. ADENEY 
taken to serve as a guide when measuring with the microscope. In this way, very 
accurate measurements were obtained, even of lines that could not possibly have been 
measured otherwise. Nearly all lines given in the accompanying tables, up to and 
including a wave-length of 2265, have been measured from grating spectra. Between 
X 2265 and X 2145*7, the very well marked lines only were measured from the grating 
spectra. The numbers not obtained from grating spectra for the other lines given in 
the tables were determined by means of an interpolation curve, constructed from 
measurements taken from prism spectra. 
The curve was laid down on millimetre paper, four millimetres being allowed for 
each difference of a tenth-metre in wave-length, and one millimetre for each ^- 0 J Q -^th 
of an inch of difference in the scale numbers. The wav T e-lengths were taken as 
normals, and the scale numbers as abscissae; the aggregate length of the curve from 
X = 4800 to X 2020 was about nine metres. The spectra were taken from one quartz 
prism of 60°, composed of two halves each of 30°, one of right-handed and the 
other of left-handed rotation. The prism was fixed for the minimum angle of 
deviation of the cadmium line 2747'7, and the photographs were similar to those 
published in the Journal of the Chemical Society (Transactions, vol. xii., p. 85, 
W. N. Hartley). 
In the tables, besides the wave-lengths, the scale numbers from the prismatic 
spectra are in every case given. These numbers are expressed in hundredths of an 
inch and fractions thereof. The numbers for the various metals are strictly 
comparable with each other, since the measurements from each spectrum have been 
reduced to a standard spectrum of an alloy of tin and cadmium. This was accom¬ 
plished in the following way. The spectrum of each metal was photographed with 
that of the tin-cadmium alloy. The same electrode of the alloy was employed for 
all the spectra, and was not moved during the time the whole series was being- 
photographed. An alloy of tin-cadmium was used because it gives a large number 
of well-defined lines, equally distributed. Notwithstanding the care taken, and that 
twelve spectra were photographed on the same plate, in only four spectra are the tin 
and cadmium lines coincident in position. The mean of the readings for these four 
spectra was taken for the standard spectrum, and all others were reduced to it 
by finding the corrections for the tin and cadmium lines, and interpolating correc¬ 
tions for lines between them. 
For the construction of the curve, 180 lines from the different spectra were 
employed. The whole of these lines were cut by the curve. A few lines were left 
a little to the one side or the other, but these are not included in the above number. 
The lines employed for the portion of the spectrum beyond the cadmium line 2146*8 
were those of zinc, M. Cornu’s numbers for their wave-lengths being made use of. 
This portion of the curve was made as continuous as possible with the other. Not all 
the points were cut by it, and to this is owing the slight difference between some of 
the numbers in our table of the zinc lines and those of M. Cornu, 
