358 On the Measurement of the | July, 
nating power have been observed ; but on placing two of these lamps 
in opposition, no such variations have been detected. The same 
candle has been used, and the experiments have been repeated at 
wide intervals, using all usual precautions to ensure uniformity. 
The results are thus shown to be due to variations in the candle and 
not in the lamp. 
It is expected that whoever may be inclined to adopt the kind 
of lamp here suggested will find not only that its uniformity may 
be relied upon, but that, by following accurately the description and 
dimensions here laid down, each observer will possess a lamp of 
equivalent and convertible photometric value; so that 1esults may 
not only be strictly comparable between themselves, but, within 
slight limits of accuracy, comparable with those obtained by other 
experimentalists. The dimensions of wick, &c., here laid down are 
not intended to fix the standard. Persons engaged in photometry 
as an important branch of their regular occupation will be better 
able to fix these data than the writer, by whom photometry is only 
occasionally pursued as a means of scientific research. Already 
many improvements suggest themselves, and several causes of varia- 
tion in the light have been noticed. Future experiments may point 
out how these sources of error are to be overcome; but at present 
there is no necessity to refine our source of standard light to a greater 
degree of accuracy than the photometric instrument admits of. 
The instrument for measuring the relative intensities of the 
standard and other lights, next demands attention. The contri- 
vances in ordinary use are so well known that a short sketch of the 
principles on which they are based need only be given. Most of 
them depend on a well-known law in optics, namely, that the amount 
of light which falls upon a given surface varies inversely as the 
square of the distance between the source of light and the object 
illuminated. The simplest observation which can be taken is made 
by placing two sources of light (say a candle and gas-lamp) opposite 
a white screen a few feet off, and placing a stick in front of them, 
so that two shadows of the stick may fall on the screen. The 
strongest light will cast the strongest shadow; and by moving this 
light away from the stick, keeping the shadows side by side, a 
position will at last be found at which the two shadows appear of 
equal strength. By measuring the distance of each light from the 
screen, and squaring it, the product will give the relative intensities 
of the two sources of light. 
In practice, this plan is not sufficiently accurate to be used 
except for the roughest approximations; and from time to time 
several ingenious contrivances, all founded upon the same law, have 
been introduced by scientific men, by which a much greater accuracy 
is obtained ; thus, in Ritchie’s photometer the lights are reflected on 
to a piece of oiled paper in a box, and their distances are varied 
