922 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHE\nCAL EESEAECHES. 
portion of the wire will illuminate the diaphragm, when its apparent area is equal to 
that of the sun, with the intensity 
1 ^ 1 . 
9^' 
The visual brightness (G) of the sun, compared with that of a brnning magnesium wire, 
is therefore 
G= 
^9 
If the radius of the opening through which the sun shines be r ; the distance of the 
opening from the photometer diaphragm d ; the radius of the incandescent surface of 
magnesium considered as circular ; and the distance of this surface from the 
diaphragm d^, the brightness of the sun compared with that of the magnesium is 
S is found from the formula 
G== 
d9r\ _ 
S^’ 
sin® ((p-— <p') tan® 
sin® (p + tan® (<p + p') ’ 
when (p signifies the angle of incidence and p' the angle of refraction of the ray. <pi is 
calculated from the refractive index of the mirror ~l-55 = ^- when the angle of 
Sill 
incidence (p) is known. The value of p is obtained from the azimuth (A) of the opening 
through which the rays pass ; from the sun’s dechnation (^) on the day of experiment ; 
from the time of observation (t), and from the latitude of Heidelberg by means of the 
formula (8.) already employed. By substituting the folio whig numerical values, 
A=73° 44' ; —17° 68'; ^=0° 0' ; y) = 49° 24' in the formula, we have 
S=0-06101. 
Dhect measurement gave 
r=0T995 millim. ; fi = 0'9725 millim. ; d!=2590 millims. ; 
Hence 
G=524-7. 
(?i=2440 millims. 
The observations were made on November 13th, 1858, at 12^‘ 0“, when the sun’s zenith- 
distance was 67° 22'. The brightness of the sun’s disk, as measured by the eye, is 
therefore, at this zenith-distance, 624-7 times as great as that of the burning magnesium 
wire, whilst at the same zenith-distance the chemical brightness of the sun is only 36*6 
times as great. 
The steady and equable light evolved by magnesium wire burning in the afr, and the 
immense chemical action thus produced, render this soui’ce of light valuable as a 
simple means of obtaining a given amount of illumination expressed in terms of our 
measurement of light. According to the above experiment, a piece of magnesium wire 
0-987 metre long and 0-297 millim. in diameter, efiects at a distance of 2-44 metres an 
action of 181-7 units of light, of which 10,000 are equal to 1 degree of light. For every 
millimetre of magnesium wire burnt, there is therefore produced 1-1 unit of light at a 
