1901 - 2 .] Professor C. Piazzi Smyth on Sodium lines. 227 
subsequently kept up through several hours, as a test that no larger 
and cumulative time-change existed. 
Most of the observations were made with the grating in the 
angular position most nearly normal to the collimator, when the 
distance from I) 1 to D 2 marks 266 of the micrometer on the second 
order of spectrum. But the negative results thence derived have 
been tested and confirmed by other observations made after turning 
the grating through its position of simple reflection, to the second 
order of spectrum on the opposite side thereof, where the distance 
of the lines is necessarily less wide, and, in fact, measured only 
243 of the micrometer. 
Again the poles of the induction coil have been changed, so that 
the direction of the illuminating current through the capillary has 
been made to pass straight to, or straight from, the slit, besides the 
light having been sent to the slit sometimes direct in the axis of 
the collimator and sometimes at right angles, by means of a totally- 
reflecting prism and an altered place of standing for the tube- 
holder ; but none of these things have had any certainly visible 
effect on the absolute spectrum-place of any line in the field of 
view. See the following observations on May 18 and May 20, as 
extracted from my observing ledger. 
[Tables. 
