TOTAL DAYLIGHT AT KEW AND PAEA. 
563 
for the same in 1866. Another important factor as influencing the transparency must 
not be overlooked, viz. the presence of finely divided solid particles which floating about 
produce the phenomenon of atmospheric opalescence. This, taken in connexion with 
the well known fact of the greater velocity of the winds in spring than in autumn, 
thus increasing the quantity of these floating particles in the spring, points to an 
explanation of the high autumnal and low vernal chemical intensity. 
Part II.— THE PAEA OBSEEYATIONS. 
Our knowledge concerning the distribution of the chemically active rays on the earth’s 
surface is as yet very limited, and any conclusions with respect to the intensity of the 
chemical rays in the tropics have been, up to the present time, based upon the vague and 
unsatisfactory statements of photographers. According to photographic observations it 
would appear that in advancing from England towards the equator the difficulty of 
obtaining good pictures is increased, and more time is said to be required to produce 
the same effect upon a sensitive film under the full blaze of a tropical sun than in the 
gloomier atmosphere of London *. It is likewise stated f that in Mexico, where the 
light is very intense, from twenty minutes to half an hour was required to produce pho- 
tographic effects which in England occupy but a minute ; and it is said that travellers 
engaged in copying the antiquities of Yucatan have on several occasions abandoned the 
use of the photographic camera and taken to their sketch-books. Dr. Draper has also 
noticed certain differences of a similar kind between the light of New York and that of 
Virginia, and hence a supposition has been thrown out of the existence of a peculiar 
retarding action exerted by the luminous and the calorific rays upon the more refrangible 
and chemically active portion of the sunlight. In order to test the validity of these 
statements, it becomes a matter of great interest to determine directly the intensity of 
the chemically active rays in the tropics. Through the kindness of Messrs. Alfred 
Booth and Co., of Liverpool, and thanks to the zeal and ability of my assistant, Mr. T. 
E. Thorpe, I have been able to obtain such a set of measurements made at Para, situated 
nearly under the equator in the northern province of the Brazils, and lying upon a branch 
of the Amazons, in longitude 48° 30' West, and latitude 1° 28' South. 
The observations, the results of which are given below, were made at Para by Mr. 
Thorpe from the 4th to the 26th of April 1866, in a situation possessing a clear horizon. 
Owing to the rainy season having set in when the experiments were commenced, the 
changes in the chemical intensity as observed from hour to hour, and even from minute 
to minute, are very sudden and remarkable, and render a large number of daily obser- 
vations necessary. These sudden changes are well represented in the curves, figs. 9, 10, 
12, and 13, Plate XXL, showing the variation of chemical intensity at Para during the 
* Golding Bird, ‘ Natural Philosophy/ p. 622, 5th Edit, 
f Eobert Hunt, ‘ Eesearches ou. Light,’ p. 366, 
4 H 
MDCCCLXVII. 
