118 Proceedings of the British Association, 



researches, that he was satisfied that the amount of chemical 

 power was not in direct ratio with the quantity of light, but that 

 at different hours of the day, and at different seasons of the year, a 

 remarkable variation may be found to exist. The peculiar conditions 

 of plants in the morning and evening and the phenomena of hyber- 

 nation were referred to variations in this chemical (actinic) power, 

 and the processes of germination, of flowering, fruiting, and the 

 autumnal decay of the leaf, shown to be, in all probability, dependent 

 upon the same influence. This being the case, it became desirable 

 that some self-registering means of remarking the changes in the 

 condition of the sun's rays should be adopted. The actinograph is 

 intended for this purpose. It consists of a cylinder of brass, upon 

 which is placed a sheet of photographic paper, so prepared with the 

 bromide of silver, that all the rays of the prismatic spectrum shall 

 act upon it with equal intensity ; over this is placed another cylinder, 

 which is carried round by a clock movement once in the twenty-four 

 hours. In this outer cylinder is a triangular opening, divided by 

 bars into a hundred parts, the smallest part of the slit being one 

 hundred times less than the largest. As this cylinder moves over 

 the paper, it is of course exposed for different periods of time to the 

 solar influence over different parts of the divisions ; one portion being 

 only exposed for one minute, whereas the largest opening admits of 

 an exposure for one hundred minutes. Thus the greatest intensity 

 of actinic power will produce, during the time of least exposure, the 

 same effect as is produced by the weakest radiations during the 

 period of prolonged exposure. The papers are removed every night 

 and divided into twenty-four parts, and we have hence an exact 

 measurer of the amount of chemical power exerted during every hour 

 of daylight ; and as the results may be numerically registered, and 

 the uncertain effects of fixing thus removed, we procure with atten- 

 tion an accurate record for any period of time. This instrument is 

 a modification of one devised by Sir John Herschel, with some 

 improvements suggested by Mr. Jordan. Mr. Hunt stated, that 

 circumstances had prevented his doing more than make a few 

 trials of the apparatus, but that he hoped after the meeting to fix it, 

 and use the actinograph for constant registration. 



