PEOFESSOE BimSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 893 
From these expenments we see that a paper diaphragm, with a transparent spot of 
stearic amd, can be most advantageously employed for the photometric measurement of 
rays of light falling upon a point at very various angles of incidence. 
In order to determine the intensity of the sky’s brightness with ease and exactitude, 
we have employed the photometer depicted in fig. 6, Plate XLIII. fig. 5, represents 
the paper diaphragm with the spot of stearine, which closes the vertical tube {b), and is 
m the same plane as the horizontal plate {c) fastened on to the tube {b). This tube {b) 
IS coloured in the inside white, and into it is soldered the narrow tube {d), black- 
ened inside, and placed at such an angle below the diaphragm («), that on looking 
through it the stearine spot on the paper is plainly seen. The observations are more 
conveniently made when a smaU blackened box {e), containing a mirror and carrying an 
eyepmce (/), is placed on the end of the tube {b), and thus the change of shade in the 
stearine diaphragm noted. The tube b is fixed into the top of the tin case Jii, one side 
of which IS open, and the lower end of the tube b is closed by a divided circle, half of 
which, as is seen in fig. 6, a < 2 , Plate XLIII., has been removed. This semicircular open- 
mg can be partially or totally closed at pleasure by a moveable disk of metal, and the 
angle thus left open read off on a divided circle. Supposing that a constant source 
of light be placed in the case hi, the lower surface of the stearine diaphragm receives 
a quantity of light which is determined by the divided scale of the semicircular opening 
at the bottom of the tube. If we suppose that the semicircular arc is opened to its 
full extent (180°), and that the amount of light which is thrown on the lower surface 
of the diaphragm is larger than that which falls on the upper surface, the spot of stea- 
rine will appear dark on a bright ground. If the opening be now gradually closed, 
the spot will be observed to become lighter, until at a certain position of the divided 
circle the spot entirely disappears, and a homogeneous white surface is seen. If the size 
of the open sector be still diminished, the spot again appears, but this time it is bright 
on a daik giound. When the divided arc has been placed in such a position that for a 
pven illummation (A) the spot of stearine disappears, any other source of light placed 
m a, which under like conditions does not cause the spot to appear, is equal in intensity 
to A. 
For the pui-pose of measuring the difliise light of day, the diaphragms are best made 
of white drawing-paper, of about double or treble the thickness of ordinary writing- 
paper. "W hen the whole of the paper, with the exception of a small ring, is saturated 
with stearic acid, the observations are more exact than when a paper with merely a spot 
of stearic acid is made use of. In order to prepare such a diaphragm, the drawing- 
paper, with a piece of filter-paper under it, is laid upon a warm metallic plate, and a 
piece of fused stearic acid (a stearine candle answers the purpose when the pure acid is 
not at hand) gTadually rubbed into the heated paper, so that a small circle in the centre 
of the paper is the only part not saturated with the stearine. When the plate is cool, a 
small piece of the acid is laid in the centre of the circle, and the plate gently warmed 
until the stearic acid has suijk into the pores of the paper, so that only a small ring not 
MDCCCLIZ. a „ 
