PEOFESSOE BimSEJf AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 915 
gives the chemical illumination for the unit of area upon which the sun’s rays fall per- 
pendicularly, In order that the supposed surface may be considered to be in the plane 
of the horizon and not at right angles to the incident ray, the values of Wo have been 
multiphed by cos <p. 
Table XX. 
Melyille 
Island. 
Eejldavik. 
Petersburg. 
Manchester. 
Heidelberg. 
Naples. 
Cairo. 
h li 
6 A.M. or 6 p.M. ... 
0*00 
0-00 
0*00 
0*00 
0*00 
0*00 
0*00 
7 a.m. or 5 p.M. ... 
0*00 
0*02 
0*07 
0*22 
0*38 
0*89 
1*74 
8 A.M. or 4 p.M. ... 
0*07 
1*53 
2*88 
5*85 
8*02 
13*31 
20*12 
9 a.m. or 3 p.M. ... 
0*67 
6*62 
10*74 
18*71 
23*99 
35*88 
50*01 
10 A.M. or 2 p.M. ... 
1*86 
13*27 
20*26 
32*91 
40*94 
58*46 
78*61 
11 A.M. or 1 p.M, ... 
3*02 
18*60 
27*55 
43*34 
53*19 
74*37 
98*33 
12 A.M 
3*51 
20*60 
30*26 
47*15 
57*62 
80*07 
105*3 
The curves (fig. 18, Plate XLVIII.) represent the illuminations at the various places 
and times given in the foregoing Table (XX.). The ordinates represent the chemical 
actions, expressed in degrees of light, effected at the hours measured on the absciss® 
for the places named on the curves. If we compare these curves with those of 
fig* 11? Plate XLYI. representing the diffuse light, the singular fact becomes apparent, 
that at the time of the vernal equinox the chemical actions which the direct sunlight 
effects from the north pole to below the latitude of Petersburg is, during the whole day^ 
less than that effected by the total diffuse daylight ; and that even in lower latitudes, 
down to the equator, the same phenomenon is observed, if not for the whole, yet for a 
portion of the day. 
A fuifher comparison of formul® (9.) and (14,), shows that not only at the vernal 
equinox, but at all times and in all places where the sun rises more than 20° 56' above 
the horizon, from sumise till it attains a given height above the horizon, the chemical 
action effected by the diffuse daylight exceeds that of the direct sunlight ; and that as 
the sun gradually rises, a point is reached at which both sunlight and diffuse daylight 
pioduce exactly the same amount of chemical action, whilst beyond this point the effeci: 
of the sunshine is most powerful. This phase of equal chemical illumination ” between 
fi9,ylight and sunshine, occurs twice daily, and rej)resents a certain zenith-distance of the 
sun (^), which may be found by a method of approximation from the formula 
2‘776-f-80‘849 cos (p — 45'996 cos^ ip= 
318-.S 
num log 
0-4758P’ 
cos <p 
where the expression to the right denotes the chemical action effected by the sunlight 
at the pressui-e P (formula 14.), and that on the left hand gives the action proceeding 
from the whole heavens (formula 9.), 
If we assume that the barometric pressure was 0“i-76, that the plane of the surface 
illuminated by the sunlight was placed perpendicular to the incident rays, and that the 
area illuminated by the daylight lay in the plane of the horizon, we find, on calculation, 
