42 THE ROYAL SOCIETY OF CANADA 
COMPARISON OF MEASURES. 
14. Before discussing the velocity and the law of its variation 
with the latitude, it is desirable to attempt an explanation of the 
systematic difference between the values of the velocity obtained by 
Plaskett and De Lury. Although the plates used are not the same, 
the difference persists when the same plates are measured by the two 
observers, as will be seen later. In the early measures of rotation 
plates the fields of the measuring microscopes were left unmasked, 
but later, as the quantity of light getting through was very fatiguing 
to the eye, diaphragms were arranged to eut out part of the illumina- 
tion. This was effected in the case of the Repsold measuring machine 
used by Plaskett by placing a thin dise with three slots cut in it in 
the eyepiece of the micrometer just above the focal plane. By this 
means only the three strips of spectra were visible, the light coming 
through on the outside and between the strips being occulted by the 
dise. In the case of the Toepfer machine used by De Lury, a single 
slit of the width of one of the spectral strips was cut in a brass plate 
which was held by a movable arm attached to the rigid microscope 
carriage close to the plate and which could be, by a convenient screw, 
readily moved transversely, so that in measuring, only one of the 
three strips of spectrum could be seen at a time. This latter arrange- 
ment was devised by De Lury to keep the configuration of the spectrum 
lines the same for each measurement, because hefound that his measures 
were affected by the configuration of the lines in the three strips; * and, 
further, to keep himself in ignorance of the magnitude and direction 
of the displacement so that his measures could in no way be affected 
by prejudice. For this latter reason also he postponed his computa- 
tions of the velocities until all the measurements of his series were 
completed. On the other hand, as only part of the line in the strip 
being measured can be seen distinctly at one time, as the eye has to 
move up and down to make the best placing of the wire, it was felt 
by Plaskett, that as the other strips could not be seen while the setting 
was being made, they could not influence his measuresin any way. Con- 
sequently the simpler expedient of a fixed diaphragm occulting only the 
extraneous light was deemed sufficient by him. This is corroborated by 
the fact that the difference between Plaskett and De Lury is practically 
constant at all latitudes (except the pole), although the relative displace- 
ment of the lines in the spectra varies widely. On the other hand, the 
measures of 15 equatorial spectra by De Lury both with and without 
the mask gave a systematically greater value for the former of 0.012 

* Jour. Roy. Astron. Soc. Can. 5, 384-407. 
