PEOFESSOE BUIS-SEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 907 
action (wj was calculated which the sun’s image would have effected at the mean time of 
observation if no chlorine and hydrogen mixture had been present. If we call \ the 
thickness of chlorine and hydi’ogen, reduced to 0° and 0“-76, contained in the transparent 
cylinder, and if we neglect, as we can do without appreciable error, the reflexion from 
the interior of the transparent cylinder, the coefficient of extinction (a) is obtained ffiom 
the equation* 
The following Table gives the values of w and Wx as found by experiment for the 
corresponding zenith-distances (<p') of the sun, and the values of oc calculated therefrom. 
The thickness of the layer of chlorine and hydrogen collected at 22° C. and 0®-757 was 
32-3 millims., giving at 0° and 0‘"-76 a thickness of 29-8 millims. 
Table XV. 
0 . 
w. 
ee,. 
61 28 
3-41 
14-03 
0-0190 
58 26 
3-72 
15-45 
0-0191 
50 20 
4*35 
18-40 
0-0194 
The mean of these numbers gives as the value of the coefficient of extinction for 
direct sunlight in the chlorine and hydrogen mixture a=0‘01923=^millim. The 
value of N is then found to be, according to formula (!.}, by help of this number, 
N=l-036. 
The experiments still require very considerable correction. The sunlight was reflected 
into our dark room by means of a Silbeemani^’s heliostat. This was so placed that its 
mirror of speculum metal lay in one horizontal plane with the opening in the window- 
shutter. In one series of experiments, the rays, after having passed through the opening, 
were again reflected by means of a steel muTor ; in the other experiments the rays fell 
without a second reflexion, in the same horizontal plane upon the insolation-vessel. In 
the case of two reflexions, a portion of the light is lost by absorption and polarization in 
quantities varying with the angles of incidence, and the angles at which the planes of 
polarization cut one another. This vaiiable loss of light must be brought into calcula- 
tion, and it is obtained by help of the follo^ving considerations. 
A ray of light is reflected from a steel mirror in the angle i (flg. 13, Plate XLIII.); 
suppose that this ray when falling on the muTor has the intensity 1, and suppose that 
the reflected ray has the intensity yp when the ray is polarized parallel to the plane of 
incidence, and the intensity ^ when the ray is polarized at right angles to this plane. 
In PoGGENDOEFF s ‘ Annalen •j’ are found two tables of Jamin’s, in one of which are 
found the values of and the other those of s expressed as functions of i. 
* PhUosophical Transactions, 1857, p. 603. f Erganzungs, Band ii. 1848, p. 445. 
