RAYS OF THE SOLAR SPECTRUM ON PREPARATIONS OF SILVER, ETC. 57 
gaseous than a vitreous absorption as their origin. As I have no reason for preferring 
one to the other set of results, I shall take a mean of the two sets, which, with the 
differences of the coordinates or intervals between the centres of the spots, run as 
follows : 
Coordinates. 
Differences. 
Spot a. . . . 
13.... 
y 
a. . .. 
e. . . . 
- 18-20 
- 26-85 
- 35-70 
- 44-60 
- 55-50 
8-65 'l 
8-65 >Mean 8-80 
8-90 J 
10-90 
If we now consider that the half of 18 2 is 9’1, we might almost be led to expect a 
subdivision of the spot a, with its train *, into others, one of which should have its 
maximum at the fiducial point Y, and another half-way between that point and a. 
And though I am not disposed to attribute much weight to the fact, it is right to 
mention that when paper prepared with Indian ink was used, and a very pale yellow 
glass interposed between the lens and the spectrum, an oval spot, having its centre 
precisely on the point Y, was actually insulated. And this experiment, it should also 
in justice be mentioned, was made prior to the assemblage and reduction of the above 
measures, and in the absence of any speculative views concerning them. 
140. The gaseous media through which the rays have reached their point of action, 
are the atmospheres of the sun and earth. The effect of the former is beyond our 
control, unless we could carry our experiments to such a point of delicacy as to 
operate separately on rays emanating from the centre and borders of the sun’s disc. 
That of the earth’s, though it cannot be eliminated any more than in the case of the 
sun’s, may yet be varied to a considerable extent by experiments made at great ele- 
vations and under a vertical sun, and compared with others where the sun is more 
oblique, the situation lower, and the atmospheric pressure of a temporarily high 
amount. Should it be found that this cause is in reality concerned in the production 
of the spots, we should see reason to believe that a large portion of solar heat never 
reaches the earth’s surface, and that what is incident on the summits of lofty moun- 
tains differs not only in quantity, but also in quality, from what the plains receive. 
141. The solid media traversed by the rays in our experiments above related, are 
crown- and flint-glass, the latter of two varieties, though most probably differing little 
in composition, viz. that of which the prism and the concave lens of the object-glass 
are formed. On substituting a crown- for the flint-prism, the spot y was insulated as 
before, but a and (3 were less evidently separated. When the achromatic lens was 
suppressed, and (still using the crown-prism) a very thin double convex crown-lens, 
of nearly equal focal length, was used to collect the rays, so as to get rid of flint-glass 
altogether, the insulation of y was much less sensible, and the separation of a and (3 
hardly to be perceived. This would go to point out the flint-glass as the origin of 
* See Plate II. fig. 2. 
MDCCCXL. 
I 
