A Manual Of Photography

156 SCIENTIFIC INVESTIGATIONS ON PHOTOGRAPHY. has given his instrument, just as we now register the degrees of the thermometer. An instrument of this kind was made by me for the British Association, and experiments carried on with it, at intervals, for some years. Many of the results were very curious, but the instrument being placed at the Observatory at Kew, the obser- vations were unfortunately discontinued. It is believed that, with an instrument properly constructed, the details of the one employed were capable of much improvement ; many very remarkable alterations in the relative chemical power of the solar radiations would be detected. From the indications I have obtained, I believe there exists a constant law of change, and that the correct expression of the phenomena is given in the following passages—the concluding summary of my Beport on this subject to the British Association at Edinburgh: "It will be evident that the question which assumes the most prominence in our consideration of these remarkable phenomena is that of the identity or otherwise of light and actinism. "Fresnel has stated that the chemical effects produced by the influence of light are owing to a mechanical action exerted by the molecules of æther on the atoms of bodies, so as to cause them to assume new states of equilibrium dependent on the nature and on the velocity of the vibrations to vhicli they are subjected. ‘‘ Arago says, it is by no means proved that the photogenic modifications of sensitive surfaces result from the action of solar light itself. These modifications are perhaps engendered by invisible radiations mixed with light properly so called, proceed- ing witli it, and being similarly refracted. " These views fairly represent the condition in whieli the argu- ment stands, and a yet more extensive set of experiments seems to be necessary before we can decide the question. It appears, however, important that we should dismiss, as completely as possible, from our minds, all preconceived hypotheses. The phenomena were all unknown when the theories of emission and of undulation were framed and accepted in explanation of luminous effects ; and it will only retard the discovery of the truth, if we prosecute our researches over this new ground, with a determination to bend all our new facts to a theory which was framed to explain totally dissimilar phenomena."