168 ELEMENTS OF LABORATORY WORK 



At the same time we must remember that ether-waves are 

 likely to be checked or absorbed in their passage through 

 various kinds of matter in varying degrees, and that, rela- 

 tively to the size and condition of the material particles which 

 meet them, there will be some waves more completely ab- 

 sorbed than others. The mutual action of ether- waves and 

 matter forms a wide field of investigation, which our past 

 work should enable us to approach. 



White light transmitted through blue glass appears blue 

 because the particles of the glass have absorbed the rest of the 

 white light and let through the blue. So it is for other 

 colours. A coloured flower absorbs a portion of the sunlight 

 incident upon it, reflecting only those waves which confer its 

 particular colour. That a difference of wave-length con- 

 stitutes variety of colour has been readily demonstrated by 

 interposing coloured transparent bodies, or changing the colour 

 of the light, in interference experiments. The distance 

 apart of the interference bands varies with various colours, 

 and gives an opportunity of comparing the wave-lengths cor- 

 responding with the different colours. The dispersion of 

 white light in passing through a prism is brought about by 

 the fact that the speed of the small waves is more checked 

 than that of the larger waves in passing through the denser 

 medium, and small waves are consequently the more refracted. 

 A reference to the diagram explaining refraction will make 

 this clear. 



The dimensions of the wave-lengths, found by experiments, 

 are of the same magnitude as we should expect to find for 

 waves originated and influenced by particles of a size consis- 

 tent with the molecular theory. Ether-waves, molecules, 

 and atoms are physical phenomena of the same order, incap- 

 able of being directly observed, but apparent to reason. The 

 nature of the movements, among particles of matter, which give 

 rise to these ether- waves remains a matter for future investi- 

 gation. So far, we can only say that it is a kind of vibration. 

 When the temperature of a body rises, the vibration increases 

 in amplitude and may change in character. A solid or liquid 

 at a very high temperature emits white light, the constant 



