18 
REPORT— 1847 » 
memoir * On the Action of Liglit upon Plants, &c.,’ Philosophical Tran*- 
actions, vol. cxxvii. 1836, “ The difficulty, however, of comparing the rolati^e 
intensity of the light transmitted by the variously coloured media, which were 
employed in my experiments, induced me to content myself with showing, that 
the effect of light upon plants corresponds with its illuroinating, rather than 
with its chemical, or calorific iiiHuence ; and to waive the more difficult m* 
quiry, whether its operation Uj»oii the vegetable kingdom exactly keeps pace 
with the increase of its own intensity.” 
In my previous reports to the BtitUh Association in 1 84-2 and 1844, 1 have 
stated the kind of examiiintion to which 1 then subjected each coloured 
screen. Many effects which have from time to time pn'sented tluinselre*> 
have convinced me of the necessity of a still more close examination of the 
order in which radiant principles permeate the nu*dia employed. I have 
therefore in every case examined with all care the illuminating, calorific and 
effects of the solar rays which have passed the jneUia eraplojed. 
The amount of light has been detennined by measuring olTthe priimalic 
rays which pass the screen. This is prcftTable to any system of meiisurirg 
whjeh depends upoji the power of the eye to appreciate cither light orahado*. 
liayng formed a wcdl-dcfinod spectrum on a white tablet, and carefully 
m^ked off tliu centre of the yi-Jlow ray as Ix-ing the point of luaximuni light, 
an e imits of each «)f the other rays, the transparent coloured mcdiuni 
was interposed and tin? amount <»f absorption ob.servcd. These examiuatiotis, 
many times repealed, were made with reference to tlie luraiunns raysonlv; 
and m the description of my experimimts, J sholl, comiidering the unabiorbcd 
Si.?;' ^presented by 100, ..xpress the amount of light actually 
pe?meItioni ^ ’"**"*'*’** 8“''® the sum of the rays measured off after 
deternn?rff^ absorption, and which have been 
enod bul^ntfo « thermometer with a bhok- 
of 8ta«s or fluid, ami by the evaporation 
Hersehel will b.> .v? Wackened paper, aa recoimnemicd bv Sir Joliu 
influences of flip srtln,. any ray I am tar from considering the thermif 
which they Liy 
tome.wecanoSvdoaUuUii^t’. “i t^’^P^nnients mi plants it appeal 
is active under the eoildiiinn^ 
in all compar:ii?:':;;?;^rTs 
vol. exxiv. 1833)I^hy”dire^uTextipi!^''’ (Pbiltwopliical Traasactioni, 
colour exerts a ven' nowerfnl by ntherob6ervere,tha| 
permeation of heat. ^ niHucnce on the conduction, radiation Mfl 
permeating the medi" **** O'" “CtinisW- 
phienomena. Dr. Daubmiv ni attention than the otiar 
nitrate of silver behind the^colnircd s?!.* "* paper washed with 
that we are notac- 
