1871.] Molecules, Ultimates, Atoms, and Waves, 469 



or smaller proportion relatively to the others with which 

 it is associated in the ultimates of those different elements. 

 There are thus none of the phenomena presented by the 

 spectrum which cannot be explained by this hypothesis, 

 while all the phenomena concur to give it support. 



There is one peculiar case that demands separate notice. 

 It is that of the rare metals erbium and didymium, the 

 spectra from which, when they are rendered incandescent 

 in the solid state, exhibit bright lines similar to those pre- 

 sented by the spectra from incandescent vapours and gases. 

 The only feasible explanation of this phenomenon appears to 

 be that a portion of the metals is thrown into the state of 

 a very dense vapour, which clings to the solid, and that it is 

 in this vapour that the bright lines are generated.* This 

 phenomenon in no degree militates against the supposition 

 of the compound nature of the chemical elements — rather 

 the reverse. 



Marvellous as are the facts and phenomena which have 

 been here brought under review, our wonder is perhaps still 

 more highly excited, when we contemplate the adaptation 

 of the eye and the optic nerve to receive sensible impres- 

 sions from the minute motions of the ethereal particles. For 

 it must be borne in mind, that it is not the wave-motion 

 (which merely serves to carry forward the transverse move- 

 ment from point to point), but the inconceivably minute 

 transverse motions performed by the ethereal particles, 

 which are themselves really effective in producing vision, 

 heat, actinism, and fluorescence. The extreme smallness of 

 those movements altogether baffles our powers of concep- 

 tion ; yet they manifest to us their existence in a very 

 palpable manner. In the case of vision the particles of the 

 optic nerve must vibrate synchronously with those of the 

 ether, and must perform the same minute movements. This 

 very minuteness, however, explains how it is that luminous 

 waves, coming from a large field of view, can find simulta- 

 neous entrance into so small a circle as the pupil of the eye, 

 and how the optic nerve can be simultaneously impressed 

 by so great a multitude and diversity of vibrations. 



There is a phenomenon, however, which is perhaps still 

 more curious, and of which the author of this paper can 

 speak from personal experience. It is that of the optic 

 nerve being excited into action by purely internal causes, 



* The probability of this explanation will be appreciated by those who 

 remember the experiments of M. Waidele on the adhesion of gases and 

 vapours to smooth surfaces, by which he so successfully explained the curious 

 images obtained in the dark by M. Moser. 



