38 Proceedings of Royal Society of Edinburgh. [sess. 
therefore be at almost, if not quite, the same temperature as that 
of the room itself, and any change in the temperature of the liquid, 
would be due chiefly to the slight though unavoidable handling, 
and to the proximity of the observer. The actual temperature of 
the water undergoing examination in the prism, was found by the 
standardised thermometer, which was used in connection with 
the density determinations already ' described, the reading being 
taken immediately after the position of minimum deviation had 
been found. The point of intersection of the cross threads in the 
telescope was in every case made to coincide with the right-hand 
edge of the image of the slit of the collimator, as it was found that 
far more concordant and trustworthy readings were obtained in 
this way, than by bringing the point of intersection upon the esti- 
mated centre of the image ; successive readings of the same quantity, 
when effected by the latter method, were sometimes found to differ 
by as much as 6" or 8" of arc, whereas by the former, or edge-of- 
slit method, the various readings rarely differed by more than 2", 
and were generally identical. 
To find the value for the minimum deviation of the D line by 
a sample of water, the following method of procedure was adopted. 
(1) The prism was washed out twice with portions of the water to 
be examined, then filled, and the thermometer inserted ; (2) the 
direct reading for the edge of the slit was made ; (3) the prism 
was placed upon the spectrometer, and the position of minimum 
deviation found; (4) the temperature of the water in the prism 
was noted ; (5) the position of minimum deviation was read ; and 
(6) the prism was removed from the spectrometer, and the direct 
reading for the edge of the slit again taken. If the direct readings 
(2 and 6) differed by more than 2" of arc, the whole process was 
repeated; this, however, was only found to be necessary in one 
instance. With a little practice the whole of the above operations 
may be performed in five minutes. 
Temperature Corrections. 
Before the values obtained for the minimum deviations or 
refractive indices could be compared with each other, it was neces- 
sary to study the effect produced upon them by a change in tern- 
