1867.] of Total Daylight at Kew and Para in 1865-1867. 



43 



1867 inclusive, liave been obtained. Curves exhibiting tbe daily rise 

 and fall for eacH of the twenty-four months, as well as a curve showing the 

 biennial variation of chemical intensity for the same period, accompany 

 the paper. The curve of yearly chemical intensity is found to be un- 

 symmetrical about the vernal and autumnal equinoxes ; thus in spring 

 and autumn the results are as follows : — • 



1866. Mean Cli. Int. 



March 34-5 



April 52-4 



September 70-1 



August 94-5 



1865-67. Mean Ch. Int. 



March 1867 30-5 



April 1865 97-8 



September 1865 107-8 



August 1865 88-9 



Or for 100 chemically active rays falling during the months of March 

 and April 1865, 1866, and 1867 at Kew there fell in the corresponding 

 autumnal months 167 rays, the sun's mean altitude being the same. 



The author discusses the probable causes of this autumnal maximum; 

 he finds that it is not due to variation in the amount of cloud, and be- 

 lieves that it is to be explained by a less amount of atmospheric opales- 

 cence in the autumn than in the spring. 



The yearly integral for the twelve mouths, January to March 1867 

 and April to December 1865, is 55*7, whereas'^that for the twelve months 

 of the year 1866 is 547. 



II. JBard Observations. 



All the knowledge we possess concerning the distribution and in- 

 tensity of the chemically active rays in the tropics is derived froin the 

 vague statements of photographers. According to their observations it 

 appears that the difficulty of obtaining a good photograph increases as 

 we approach the equator ; and more time is said to be needed to pro- 

 duce the same effect upon a sensitive plate under the full blaze of a 

 tropical sun than is required in the gloomier atmosphere of London. 

 Thus in Mexico, where the light is very intense, from twenty minutes 

 to half an hour is stated to be required to produce photographic effects 

 which in England occupy but a minute. Hence the existence of a 

 peculiar retarding influence has been suggested which the heating and 

 luminous rays are supposed to exert upon the more refrangible portions 

 of the spectrum. The fallacy of these statements has been fully proved 

 by a series of direct measurements of the chemical intensity of sunlight 

 under the equator, made at Para by Mr. T. E. Thorpe. The curves of 

 daily chemical intensity given in the paper show that the activity of the 

 chemical rays in the tropics is very much greater — on one day fifty-five 

 times as great, as in our latitudes ; and these measurements prove that 

 the reported failures of photographers cannot at any rate be ascribed 

 to a diminution in the chemical intensity of sunlight. The following 

 numbers give some of the daily mean chemical intensities at Para com- 

 pared with the same days in Kew : — 



