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LIV. New Standards of Optical Wave-length. 
By Prof. H. Kayser, Bonn* 
NHE Section A of the British Association has for so many 
years promoted the knowledge of spectroscopy, that I 
think it well to bring before the Section a question which 
becomes more and more urgent for the progress of spectro- 
scopy in both the theoretical and practical applications, viz., 
a more accurate system of wave-lengths. 
For many years spectroscopists had to be content with the 
determinations made by Angstrém for the visible part of the 
spectrum, and extended later by Cornu to the ultra-violet 
part as far as 8000 A. For the wave-lengths to 2200 A, 
Liveing and Dewar, and Hartley, gave the first measure- 
ments. At that time errors between 1 and 5A were not 
infrequent, 7. e. wave-lengths could be determined to only 
about one one-thousandth of their value. : 
An immense advance was made by Rowland. He no 
longer tried to determine absolutely the wav e-lengths of a 
oreat many lines by the grating, but, applying the much 
more accurate method of coincidbaies. found the values of 
some hundred lines between 2000 am 7600 A, relative to a 
value of the D-lines taken somewhat at random from the 
absolute measurements by Bell, Kurlbaum, Miiller and Kempf, 
Pierce. The method of coincidences, as is well known, is 
founded on the theory of the concave grating, by which at 
the same point a line of wave-leneth X in the - fret order, of 
4X in the second order, of 2X in the third, &e., are brought 
= focus. 
Rowland expressed the opinion that none of his standards 
could deviate by a hundredth of an A from the correct value; 
and this seemed probable when one considered the first-rate 
instruments used by Rowland, and the eminent ability of 
Rowland and his assistant Jew a for such work. 
This faith received the first shock when Michelson, by his 
beautiful interferometer method, determined at Paris, in col- 
laboration with Benoit, the absolute values of three Cd lines. 
Not only were the absolute values greatly different from Row- 
land’s,butalso the relative values showed discrepancies amount- 
ing ia more than 0°03 A. Rowland’s system received a much 
severer blow when Profs. Fabry and Perot, by another kind 
of interferometer, measured many iron and solar lines in the 
* Read before the British Association at Cambridge, August 22, 1904. 
Communicated by Prof. Larmor, Sec.k.S. 
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