302 



Messrs. Roscoe and Thorpe on the Relation [Mar. 31, 



chemical intensity. The method of measurement adopted was that de- 

 scribed in a previous communication to the Society*, founded upon the 

 exact estimation of the tint which standard sensitive paper assumes when 

 exposed for a given time to the action of daylight. The experiments were 

 made as follows : — 



1 . The chemical action of total daylight was observed in the ordinary 

 manner. 



2. The chemical action of the diffused daylight was then observed by 

 throwing on to the exposed paper the shadow of a small blackened brass ball, 

 placed at such a distance that its apparent diameter, seen from the position 

 of the paper, was slightly larger than that of the sun's disk. 



3. Observation No. 1 was repeated. 

 4i. Observation No. 2 was repeated. 



The means of observations 1 and 3 and of 2 and 4 were then taken. The 

 sun's altitude was determined by a sextant and artificial horizon, im- 

 mediately before and immediately after the observations of chemical in- 

 tensity, the altitude at the time of observation being ascertained by 

 interpolation. 



It was first shown that an accidental variation in the position of the brass 

 ball within limits of distance from the paper, varying from 140 millims. to 

 230 millims., was without any appreciable effect on the results. One of 

 the 134 sets of observations was made as nearly as possible every hour, and 

 they thus naturally fall into seven groups, viz. : — 



(1) Six hours from noon, (2) five hours from noon, (3) four hours from 

 noon, (4) three hours from noon, (5) two hours from noon, (6) one hour 

 from noon, (7) noon. 



Each of the first six of these groups contains two separate sets of observa- 

 tions, — (1) those made before noon, (2) those made after noon. It has 

 already been pointed outf, from experiments made at Kew, that the mean 

 chemical intensity of total daylight for hours equidistant from noon is the 

 same. The results of the present series of experiments prove that this con- 

 clusion holds good generally ; and a Table is given showing the close ap- 

 proximation of the numbers obtained at hours equidistant from noon. 



Curves are given showing the daily march of chemical intensity at 

 Lisbon in August, compared with that at Kew for the preceding August, 

 and at Para for the preceding April. The value of the mean chemical 

 intensity at Kew is represented by the number 94*5, that at Lisbon by 

 110, and that at Para by 313\3, light of the intensity 1 acting for 24 hours 

 being taken as 1000. 



The following Table gives the results of the observations arranged 

 according to the sun's altitude. 



* Roscoe, Bakerian Lecture, 18G5. 



t Phil. Trans. 1867, p. 558. 



