measuring the comparative Intensities of Light. 79 
otherwise arise from reflected light, and from the presence of 
too great a number of visible objects. 
In order to move the lights to and from the photometer 
with greater ease and precision, I have provided two long and 
narrow, but very strong and steady tables, in the middle of 
each of which there is a straight groove, in which a sliding 
carriage, upon ydiich the light is placed, is drawn along by 
means of a cord which is fastened to it before and behind, and 
which passing over pullies at each end of the table, goes round 
a cylinder, which cylinder is furnished with a winch, and is so 
placed near the end of the table adjoining the photometer, 
that the observer can turn it about, without taking his eye 
from the field of the instrument. 
Many advantages are derived from this arrangement ; as 
first, the observer can move the lights as he finds necessary, 
witnout the help of an assistant, and even without removing 
his eye from the shadows ; secondly, each light is always pre- 
cisely in the line of direction in which it ought to be, in order 
that the shadows may be in contact in the middle of the verti- 
cal plane of the photometer ; and thirdly, the sliding motion of 
the lights being perfectly soft and gentle, that motion pro- 
duces little or no effect upon the lights themselves, either to 
increase or diminish their brilliancy. 
These tables, which are 10 inches wide and 35 inches high, 
and the one of them 12 feet, and the other 20 feet long, are 
placed at an angle of 6o° from each other, and in such a situa- 
tion with respect to the photometer, that lines drawn through 
their middles in the direction of their lengths, meet in a point 
exactly under the middle of the vertical plane or field of the 
