TRANSACTIONS OF THE SECTIONS. 
37 
Tie solar spectrum is therefore endowed with three different photogenic actions, 
mdtiree different destroying actions corresponding to the red, yellow and blue rays. 
He three photogenic actions of the spectrum thus divided have distinct charac- 
each radiation is endowed with a photogenic power peculiar to itself, and which 
ZK to the Daeoerreotype plate an affinity for mercurial vapours; nevertheless these 
i!c« actions are so different that wp cannot by mixing them artiicially make them 
aiia each other, as thev are antagonistic. The effect produced by the blue rays is 
iaoved by the red and'yellow, tlic effect of the red is destroyed by the yellow, and 
4«of the yellow by the red. and the effect of the two latter is destroyed by the 
Ue alternate change of the surface of the plate by these three radiations to 
pwtthat the chemical compound remains always the same under these ihfforent 
diaices, and that there is no separation or disengagement of the constituted ele- 
■W; and the destroction by one ray of the effect produced by another, restoring 
Ih plate to its former sensitiveness, likewise favours this conclusion.^ 
D«tricity, which probablv accompanies each miliation, acts positively uncler the 
iArace of one, and negatively under the other, without changing the chemical 
aopound. In one case this influence would give an affinity for mercury, ana in the 
it would destroy it. 
0« a proposed OpliraJ Experiment. Ey Mr. Dale. 
At the last meeting of the Association Mr. Dale proposed an cxperimei^al method 
ffcterniining a point in physical optics on which some niathematicinns have enter- 
a different view from that of Fresnel, Airy and Cauchy ; whether, namely, m 
polarized ray the vibration is parallel to the plane of polarization instead o 
pcipondicular to it, a« suppased bv the philosophers named. _ , • 
Tlie object of this note is to explain tliat he Ims since found the experiment is not 
*«'.hvmg been made by Fresnel in 1819. (Annales de Chimie et flc f 
p. 384 ; and Annalca dc Ch. et Pb., tome xvii. p. 333. Thud bencs.) And 
'^?h he does not draw from it explicitly any inference as lo the direction ot 
JratioDfi, yet it is conceivable that the result may have influenced tlie lormation oi 
‘“‘opinioD. 
Changes in the Position of the TraiisU-instrument attributed to the Tern- 
of the EartK front the Observations of ProfesRor C. ^ ^ 
Mirtburgh, Communicated by the. llov. Professor Powell, 9’C* 
^r. Mallet, in an address to the Geological Society of Dublin journal, No. 2» 
March 11 . 1846). mentioned that Sir W. R. Hamilton | 
IWmo/ Iml in the transit initruments at his observatory ; Dr- At,!/ 
^•Aug, 1820) had aUo found such a change both In the general level of the obRcr- 
at Armagh and also a motion in azimuth, recurring at anniid ^ 
!!^ntly dependent on the temperMure of the earth. Prof. C. 
^"Dch observations in detail at the observatory on the Calton Hill, ' 
^ by the thermomclric deterroinatioiia of the changes of the ti-inpcratur 
J^nt soil made under tlie direction of Prof. Forbes by are 
««d. The movements both in the level of the transit and aUo in 
graphically in curv-es. and exhibit a remarkable agreement the changes 
0 ■western end of the axis rising and deviating tow “ , 
“' But by a discussion of the thermnmctrical indications Jt depths vary 
one inch to twenty-four feet, the earth's tcmperalnre 
of these anomalies, and the author's roDclusion 
of instrument are due to 
on the brass supports of the axis, in which a bad pnnaple of construction 
jostment of the level is employed. 
The 
On Anoninfies in the Dispersion 
Me Rev. Professor Po'VELL, • 
relation between the refractive indices in any medium for w i e ig 
