1862.] 311 



The next subject considered is the preparation of an unvarying 

 tint which can be easily obtained and used as the standard of com- 

 parison. This is effected by grinding together 1000 parts of pure 

 oxide of zinc with 1 part of pure lamp-black. A series of experi- 

 ments showed that a colour can thus be prepared which possesses a 

 constant and unalterable shade ; and this can be used as a measure 

 of the standard tint. 



Having proved that a standard photographic paper of constant 

 sensitiveness, and a standard tint of unvarying shade can be prepared, 

 it is only necessary to apply the proposition that equal products of 

 the intensities of the light into the times of insolation effect equal 

 shades of blackness, in order to found a method of comparative 

 measurement of the chemical action of the total daylight. As the 

 unit of measurement, the authors propose to adopt that intensity of 

 the light which in one second produces the standard tint of blackness 

 upon the standard paper. 



When the standard paper is insolated in the pendulum-apparatus, 

 a strip is obtained which is tinted with every gradation of shade 

 from dark to white. If the point on this strip is determined which 

 coincides in shade with a paper covered with the standard tint, we 

 have only to look into the Table to obtain the time of insolation (), 

 in seconds, which is necessary to produce the shade corresponding to 

 the reading on the millimetre scale. If this time of insolation were 

 found to be one second, the intensity of the light then acting would 

 be 1=1 ; for any other time the intensity of the chemical rays 



would be 



As an example of such measurement, the authors append three 

 series of observations, giving the total amount of chemically active 

 rays falling on a horizontal surface at Manchester in summer and 

 winter, made at intervals of 10 minutes throughout three separate days. 

 These observations are likewise graphically represented as curves, 

 which show maxima and minima exactly corresponding to the appear- 

 ance and disappearance of the sun ; and from them some idea may 

 be formed of the vast differences which occur in the intensity of the 

 chemical rays falling on the earth's surface during the longest arid the 

 shortest days. 



In conclusion the authors state that it is possible, by using the 



