482 - BEPOET — 1889. 



ment in this direction was made rather more than a hundred jears ago by Scheele, 

 the Swedish chemist, who foimd that when chloride of silver was exposed to light 

 chlorine was given off. It was not till well in the forties that any special attention 

 was o'iven to the action that light had on a variety of different bodies; and then 

 Sir John Herschel, Robert Hunt, Becquerel, Draper, and some few others carried 

 out experiments which may be termed classical. Looking at the papers which 

 Herschel published in the ' Philosophical Transactions ' and elsewhere, it is not too 

 much to say that they teem with facts which support the grand principle that with- 

 out the absorption of radiation no chemical action can take place on a body ; in 

 other words, we have in them experimental proofs of the law of the conservation 

 01 energy. Hunt's work, ' Researches on Light,' is still a textbook to which scien- 

 tific photographers refer, and one is sometimes amazed at the amount of experimental 

 data which is placed at our disposal. The conclusions that Hunt drew from his 

 experiments, however, must be taken with caution in the light of our present 

 Itnowledge, for they are often vitiated by the idea which he firmly held, that radiant 

 heat, light, and chemical action, or actinism, were each of them properties, instead 

 of the effects, of radiation. Again, we have to be careful in taking seriously the 

 experiments carried out with light of various colours when such colours were pro- 

 duced by absorbing media. It must be remembered that an appeal to a moderately 

 pure spectrum is the only appeal which can be legitimately made as to the action of 

 the various components of radiation, and even then the results must be carefully 

 weighed before any definite conclusion can be drawn. No photographic result can 

 be considered as final unless the experiments be varied under all the conditions 

 which may possibly arise. Coloured media are dangerous for enabling trustworthy 

 conclusions to be drawn, unless the character of such media ha%"e been thoroughly 

 well tested and the light they transmit has been measured. An impure spectrum 

 is even more dangerous to rely upon, since the access of white light would be sure 

 to vitiate the results. 



Perhaps one of the most puzzling phenomena to be met with in photography is 

 the fact that the range of photographic action is spread over so large a portion of 

 the spectrum. The same difficulty of course is felt in the matter of absorption, 

 since the one is dependent on the other. Absorption by a body we are accustomed, 

 and indeed obliged by the law of the conservation of energy, to consider as due 

 to the transference of the energy of the ether wave-motion to the molecules and 

 atoms comprising the body by increasing the vibrations of one or both. 



In the case where chemical action takes place we can scarcely doubt that it is 

 the atoms which in a great measure take up the energy of the radiation falling on 

 them, as chemical action is dependent on the liberation of one or more atoms 

 from the molecule, whilst, when the swings of the molecules are increased in am- 

 plitude we have a rise in temperature of the body. I shall confine the few remarks 

 I shall make on this sufjject to the case of chemical action. The molecule of a 

 silver salt, such as bromide of silver, chemists are wont to look upon as composed 

 of a limited and equal number of atoms to form the molecule. When we place a thin 

 slab of this material before the slit of the spectroscope we find a total absorption in 

 the violet and ultra-violet of the spectrum, and a partial absorption in the blue 

 and green, and a diminishing absorption in the yellow and red. A photographic 

 plate containing this same salt is acted upon in exactly the same localities and in 

 the same relative degree as where the absorption takes place. Herej then, we have 

 an example of, it may be, the vibrations of four atoms, one of which at least is 

 isochronous or partially so, with the waves composing a large part of the visible 

 spectrum. The explanation of this is somewhat obscure. A mental picture, how- 

 ever, may help us. If we consider that, owing to the body acted upon being a solid, 

 the oscillations of the molecules and atoms are confined to a limited space, it pro- 

 bably happens that between the times in which the atoms occupy, in regard to one 

 another, the same relative positions, the component vibrations of, say, two of the 

 atoms vary considerably in period. An example of what I mean is found in a 

 pendulum formed of a bob and an elastic rod. If the bob be made to vibrate in the 

 usual manner, and at the same time the elastic rod be elongated, it is manifest that 

 we have a pendulum of ever-varying length. At each instant of time the period 



