181 
of Edinburgh, Session 1869-70. 
vibrations, the word vibration being understood in its most general 
sense as meaning oscillation between opposite phases or conditions, 
a fact revealed to us by the phenomena of interference. Even at 
this point, however, the hypothesis which forms the basis of the 
undulatory theory cannot fail to present itself to our minds, the 
hypothetical part being not so much the existence of a medium, or 
the propagation of vibrations, but the nature attributed to these 
vibrations, viz., that they consist in mere mechanical action, in- 
volving nothing but variations of pressure and displacement among 
the particles of which the medium is composed, and propagated 
according to the same laws as in ponderable media with which we 
are more familiar. The suspension of interfering vibrations is 
interpreted in the simplest manner as the result of the simultaneous 
application of equal and opposite forces, or according to a fiction 
easily understood, the superposition of equal and opposite motions, 
and their reappearance after separation as the natural consequence 
of the indestructibility of force. Moreover, our experience does 
not enable us to conceive any other kind of vibrations decomposable 
in the same manner, though the phenomenon of electrolysis seems 
to indicate the propagation of a periodic oscillation between opposite 
phases of decomposition and recomposition, involving something 
more than variations of pressure and displacement among the 
particles of water. Even the small degree of uncertainty that may 
remain at this stage of the inquiry, is diminished by the pheno- 
menon of diffraction, and by the physiological analogy between the 
eye and the ear, both of them situated like feelers of the brain ; we 
know the variety of perceptions that are communicated to the 
mind by the effect of mechanical vibrations upon one of these 
organs. 
Adopting the hypothesis, we call these vibrations waves, from 
their analogy to the vibrations so designated in the case of water, 
and the distance A. above mentioned we call the length of a wave 
of light. In order to effect its measurement, we produce the pheno- 
menon of interference ; that is done most directly by deflecting two 
pencils of light proceeding from the same source in such a way 
that they may be superposed after traversing paths differing by 
a 
2 A 
VOL. VII. 
