168 L. Bell—Absolute Wave-length of Light. 
with glass gratings has been as yet completed, but since the 
relative wave lengths, which are intrinsically of far greater im- 
portance, are now ready for publication and have been reduced 
by the value herein given, the result is published, leaving 
for further work with speculum metal gratings, its final con- 
firmation or correction. 
This portion of the determination is delayed awaiting better 
facilities for carrying it out, but the writer intends undertaking 
it at the earliest possible moment and hence leaves for a future 
paper the complete discussion of the problem. 
The writer desires here to express his deep obligations to 
Prof. Rowland, under whose guidance the work has been 
carried on and to whom a very important correction is due, 
and to Profs. W. A. Rogers and C. S. Peirce for information 
given and courtesies extended. 
EXPERIMENTAL. 
The determination of absolute wave-length involves two 
quite distinct problems—first, the exact measurement of the 
angle of deviation of the ray investigated, and second, the meas- 
urement of the absolute length of the gratings used. Hach por- 
tion of the work involves its own set of corrections, frequently 
quite complicated and difficult, but it is the latter part that is 
peculiarly liable to errors, which will be treated in detail fur- 
ther on. As to the former part, several important questions 
arise at the very outset. First is the choice between transmis- 
sion and reflection gratings. The principal work heretofore 
has been done with the former, but metallic gratings possess 
certain advantages, notably from the ease with which their 
temperature can be accurately measured, and the fact that they 
can easily be made of a size much larger than glass gratings, 
and consequently a small inaccuracy in measuring them involves 
much less error in the result. 
On the other hand the coefficient of expansion of speculum 
metal is more than twice as great as that of glass, and being a 
good conductor it is far more sensitive to small changes of tem- 
perature. And this property increases the liability to irregu- 
larities in the ruling, particularly in large gratings which require 
several days for completion. In ruling on glass change of tem- 
perature is less serious but this advantage is more than offset 
by the faults caused by the wearing away of the diamond 
point, which breaks down so rapidly that it is enormously diffi- 
cult to produce a glass grating free from flaws and at all com- 
parable in optical excellence with those upon speculum metal. 
The determination of absolute wave-length should rest on meas- 
urements made with both classes, and with sufficiently exact 
instruments and very careful experimentation, the better results 
