ON THE ACTINOGRAPH. 31 
aries; produce important modifications in the functions of the brain :—in 
_ short, the injection of various substances into the arteries and veins enables 
us to modify all the most important functions of the body ; and this, as before 
stated, by reagents, the laws of whose action we can fairly hope to discover. 
My reason for having neglected the closer investigation of these interesting 
_ phzenomena, was a determination fully to establish the law of the analogous 
action of isomorphous substances. ‘This having been accomplished, I shall 
now direct my researches to the elucidation of these secondary questions. 
Report on the Actinograph. By Mr, Rosprertr Hunt. 
If will be remembered that in 1838 Sir John Herschel proposed an instru- 
_ ment for the purpose of registering the variations of the actinic or chemical 
rays, and published in the Philosophical Transactions a design for what 
he termed an Actinograph, by which it was thought both the chemical action 
of the direct solar rays and of the diffused daylight would be registered. 
_ Dr. Daubeny, Prof. Nichols and Mr. Thomas Jordan have severally designed, 
_ and I believe used, instruments somewhat similar, but it does not appear that 
any very satisfactory results have been obtained by either of these inquirers. 
At the York meeting I pointed out the importance of some such registration, 
and at the request of the committee I had an instrument constructed, which 
 Texhibited to the Association at the Cambridge meeting. The actinograph 
_ Ihave been using differs but little from that proposed by Sir John Herschel, 
a modification of Mr. Jordan’s being introduced, by which it was thought 
_ the results could be tabulated for every hour of the day. As the form of 
_ this instrument is published in the Report for 184.5, it will be unnecessary to 
do more than describe such alterations as have suggested themselves during 
i the past year. The triangular slit, divided into one hundred parts, has been 
abandoned, it being found in practice almost impossible to discriminate be- 
_ tween the amount of coloration produced on the paper during an exposure of 
three minutes or six; consequently it became quite idle to attempt to register 
by this plan to the degree of nicety which it was hoped might be attained. 
__ A new external cylinder has therefore been constructed, in which are thirteen 
holes, commencing with a mere pin-hole and gradually increasing to a $_ 
inch diameter. By this means thirteen bands are marked upon the sensi- 
tive paper, each one separated from the other by an unaltered line, and it 
__ becomes easy to distinguish with considerable accuracy between the tints 
_ thus produced. 
Bromide of silver was the material which, from the circumstance that all 
_ the rays of the prismatic spectrum exert some influence upon it, was em- 
_ ployed in procuring most of my registrations. It has however been found 
% that under all circumstances this preparation is too sensitive, and that although 
- in the winter, when the solar radiations are weak, it answers admirably, yet 
in the bright sunshine of summer it assumes too great a degree of darkness, 
in even diffused daylight, and during the shortest exposure to which it is ex- 
_ posed during the revolution of the’cylinder. Many experiments have been 
‘made with other photographic preparations, and the result has been in favour 
: of the general use of the ammonio-nitrate of silver. It is true that this paper 
_ is not impressed by all the rays of thespectrum, but, as it is acted upon by all 
_ the rays beyond the yellow ray, and as the influence of the actinic principle 
_ throughout the entire range of the spectrum is, as it appears, always equally 
