266 Proceedings of the Royal Society of Edinburgh. [Sess. 
principle having a “ glass-wedge wheel ” immediately in front of the 
collimator slit (as described in the paper preceding this), and it was found 
to be both sensitive and convenient in use. To make a measurement, the 
observer moves backwards and forwards a small trolley P running on rails 
arranged just under his hand, which, being coupled by a cord to the other 
trolley N 2 carrying the Nernst lamp, has the effect of altering the intensity 
of the comparison beam from G 2 , and in this way a position can be found 
for P such that the intensities from the two plates are made equal. When 
this is brought about, a fountain pen attached to P is depressed, thus 
leaving a record of the position by means of a dot of ink on a long strip 
of card fastened between the rails of P. The distances of these dots of ink 
from a fiducial line drawn on the card are equal to the distances of N 2 from 
G 2 , and thus form a permanent record of the intensities ; the latter being, 
of course, inversely proportional to the squares of the distances N 2 G 2 .* 
Eight successive settings are always made and recorded, and the 
eighth depression of the pen produces an audible signal, so that the 
observer does not require to keep count. After each record the pen 
automatically moves slightly from right to left, so as to avoid the possi- 
bility of getting two dots superposed. The record of each complete 
observation therefore consists of a row of eight dots extending almost in 
* N 2 G 2 was in all cases kept large enough for this to be very approximately true. 
