300 Actinism and Magnetism. [July, 



silver consists of an ultimate of silver, retaining by a strong 

 attraction, in close proximity to itself, an ultimate of iodine, 

 there being, however, at their nearest points, a minute space 

 filled with the luminiferous ether in a highly compressed 

 condition. Let it be further assumed that, in this interval, 

 the two ethereal fluids subsist in a state of partial separation 

 — the one being accumulated next the silver ultimate by 

 reason of its being less repellent towards silver, the other 

 next the iodine ultimate by reason of its being less repel- 

 lent towards iodine. Call the former of the two ethereal 

 fluids parargyrine, and the latter pariodine. Suppose that, 

 in the system of luminous waves in a beam of light ap- 

 proaching this molecule, there are certain of the waves 

 whose vibrations are synchronous with those which the 

 silver ultimate tends to assume, and certain others whose 

 vibrations are synchronous with those which the iodine 

 ultimate performs in its tremors. The result will be that 

 the silver ultimate will begin to vibrate against the iodine 

 ultimate, and the two will alternately approach and retire. 

 This, however, they cannot do without promoting a re- 

 admixture of the parargyrine with the pariodine in their 

 normal proportions. Such a re-admixture again cannot 

 take place without weakening the attraction between the 

 silver and iodine ultimates ; for the silver ultimate begins 

 to be urged by the repulsive energy of the pariodine, which 

 is for it greater than is that of the parargyrine ; while the 

 iodine ultimate becomes exposed to more of the repulsive 

 energy of the parargyrine, which is for it greater than is 

 that of the pariodine. The consequence will be that, at 

 the moment when by the vibration the ultimates of silver 

 and iodine are farthest apart, the weakening of the attrac- 

 tion between them will be considerable. If now there be 

 introduced another chemical molecule or ultimate having a 

 strong attraction for the iodine, the probabilities are great 

 that, at the moment when the vibration attains its extreme 

 amplitude, the iodine will permanently leave the silver, and 

 attach itself to the introduced molecule or ultimate for 

 which it has, at that particular moment, a more powerful 

 attraction. 



On this general principle may be explained the develop- 

 ment of the latent image, both in the case of the Daguerreo- 

 type, and on that of the collodion film. In either case 

 it is needful to assume that the vibrations established by 

 the action of the incident light continue for a considerable 

 time after exposure. 



In the Daguerreotype, the effect, of the light appears to 



