226 Proceedings of Royal Society of Edinburgh. [sess. 
experiment with sodium lines rendered luminous in an end-on 
gas-vacuum tube by electric induction sparks. The lines so pro- 
duced were not quite so bright as I could have wished, chiefly 
owing to the small size of my galvanic battery of induction coil, 
whose sparks were only one inch long ; but the definition of the 
lines was as perfect as could be desired, and fully worthy of the 
high fame of the Rutherfurd grating. The distance D 1 to D 2 
measured, in the second order of spectrum of that grating, 266 
units of the micrometer, with an average error on each occasion of 
not more than two of those divisions ; and the final conclusion 
derived from three different methods of trying the experiment at 
eight points equally distributed round the azimuthal circle, was, — 
that there is no change of spectrum-place in a sodium line, depend- 
ing on the spectroscope looking in any one azimuth rather than 
another, to the amount of g-Jo- of the distance between D 1 and D 2 . 
Wherefore, if theory continues to assert, on its own very secure 
grounds, that there is some effect of that kind, it must be so small 
that the next search for it must be conducted with a very much 
more powerful apparatus than anything I possess. That is, if I 
made no mistakes in trying the case ; and in order that that may 
be fully judged of by others, I append herewith the original obser- 
vations, together with the following few words on the principles 
followed in summing them up, viz. : — 
As all the electrically illumined series herein alluded to were 
made by the method of absolute place on one spectral image alone, 
it became necessary to guard against any other source of disturb- 
ance to absolute place than the problematical one inquired about, 
while these observations were going on. Except when some positive 
change was made by hand, and in that case duly noted, to some 
part of the apparatus capable of altering the micrometer zero, there 
was nothing that occurred to the visible spectrum-place of any 
spectral line, except to exceedingly small amounts, and in a 
manner that allowed the assumption, and consequent correction in 
a second column, that it varied|as the time. The duration, there- 
fore, of any whole series of measures (as starting from any given 
azimuthal point, and returning to it again after passing through 
360°) was made as short as possible, >iz., five to seven minutes; 
while continued observations, in one and the same azimuth, were 
