142 Mr. Ritchie on a new photometer, &c. 
it would not be sensibly acted upon by invisible coloric, or 
heat. 
In short, I was led to the invention of a photometer, which 
appears to be the most accurate and delicate which has yet 
been described. But though such were the theoretical views 
which led to the invention of the instrument, its perfection 
does not depend upon any peculiar theory of light and heat. 
It is founded on the axiom, that equal volumes of air are 
equally expanded by equal quantities of light, converted into 
heat by absorption by black surfaces : and also on the well 
established principle that the quantity of light diminishes as 
the square of the distance of the luminous source from the 
object on which it is received. 
2. The instrument consists of two cylinders of planished 
tin plate from 2 to lo or 12 inches in diameter, and from a 
quarter of an inch to an inch deep. One end of each cylinder 
is inclosed by a circular plate of the same metal soldered 
completely air tight, the other ends being shut up by cir- 
cular plates of the finest and thickest plate glass, made 
perfectly air tight. Half way between the plates of glass 
and the ends of the cylinders, there is a circular piece of 
black bibulous paper for the purpose of absorbing the light 
which permeates the glass, and instantly converting it into 
heat. 
The two cylinders are connected by small pieces of ther- 
mometer-tubes which keep them steady with their faces 
parallel to each other, but turned in opposite directions, and 
also serve to make the insulation as complete as possible. 
The chambers are then connected by a small bent tube in 
