254 PROFESSOR L. BECKER ON 
more than were actually measured of the minima, or reversals, which were present in 
the spectrum during the whole period. On the plates taken between August 1901 and 
January 1902 the structure of the bands, including the minima, is easily seen, and in 
some bands it is visible to the unaided eye. The intensity of the spectrum between 
every two points measured was estimated on an arbitrary scale, the estimated ‘“ degrees” 
of intensity increasing with the intensity. In this paper the “intensity of the 
spectrum” stands for the intensity of blackness on the negative, while ‘“‘intensity of 
radiation” is used for the intensity of light in the focal plane of the spectrograph. I 
measured each plate about four times, alternately in opposite directions. Lach series 
includes a number of settings on the lines of the comparison spectrum, iron-calcium 
until September, and iron-titanium afterwards. The points measured on the same 
plate were then identified by a graphical process, and all those were discarded which 
had not been repeatedly observed. Only on plate No. 5 I made an exception, where, 
after the discussion was finished, I mcluded two minima which had only once been 
measured. Since the measuring occupied about half a year, and no measurements were 
taken after the reductions were begun, the results of the different plates may be con- 
sidered independent. Jor each plate I reduced each series of measurements separately 
to wave-leneths, and then combined them to mean values. The tables used in the 
reductions give the position of the micrometer screw of the measuring apparatus, re- 
ferred to an arbitrary zero, for each wave-length at an interval of 1 tenth-metre; they 
are based on Ketteler’s formula of dispersion,* and were prepared for the angles of in- 
clination at which the plates were exposed. 
The comparison spectrum determines the correction curve of the zero of the table. 
THE SPECTRUM IN MarcH AND APRIL 1901. 
4. Results of Measuwrements.—The results derived from the measurements made on 
the photo-plates Nos. 1 to 7 are given in Table I. The first column contains the mean 
wave-length, the second the average difference of one measurement from the adopted 
mean value, and the third, under the heading “ Intensity,” the estimated. degrees of 
intensity. The notation |} indicates that the intensity gradually changes from degree 
SON TA 
* Annalen der Physik und Chenve, 1881, 12. 
