316 ON THE RELATION BETWEEN SUN’S ALTITUDE AND CHEMICAL INTENSITY. 
chemical action effected, (1) by sunlight, (2) by diffused daylight on the horizontal unit 
of surface situated at various localities on the earth’s surface. 
A comparison of the unit of measure there taken can unfortunately not be made with 
that used in the Lisbon experiments ; but if we reduce the observational results for Lisbon 
(lat. 38° 40' N.) and the calculated one for Naples (lat. 40° 52' N.) to a common measure 
by assuming that the action of the direct sunlight at noon is equal in both cases, and 
reduce the other points in the Naples curve in the same ratio, we obtain the two curves 
B, B', fig. 4, whilst the corresponding curves for Lisbon are given on fig. 4, A, A'. 
The correspondence between the results of the measurements above described and 
those made by a totally different method is further shown by the close coincidence of 
the “ phases of equal chemical illumination” for sun- and diffuse-light as determined by 
both methods. In a former communication* it has been shown that in all places where 
the sun rises to a height of more than 20° 56' above the horizon the chemical action 
effected by the diffuse daylight exceeds that of the direct sunlight at first ; and that as 
the sun gradually rises a point is reached at which both sunlight and diffuse daylight 
produce exactly the same amount of chemical action, whilst beyond this point the effect 
of the sunshine is more powerful. This phase of equal illumination was calculated from 
theoretical considerations, and the result was confirmed by experiment, the difference 
between the calculated and the experimental points of equality amounting in mean to 
about thirty-five minutes. By reducing the chemical intensity of the direct sunlight 
in Table III. to that which the sunlight would produce on a plane perpendicular to the 
incident rays, we find that the phase of equal chemical intensity in reality is one which 
lasts for some time, that it begins near the calculated altitude of 18° 48', but that it 
continues for about an hour. When, however, the sun reaches an elevation of 35°, 
the intensity of the sun’s perpendicular rays becomes greater than that of the total diffuse 
light acting on a horizontal surface. 
* Bunsen and Roscoe, “Photochemical Researches. — Part IV.,” Phil. Trans. 1859, p. 915. 
