The movement of the emitters of the spectral lines may be deduced on the basis of the Doppler principle.

The question of the composition of perceptible objects is one which already occupied the mind of the ancient Greeks.

The emitters of the spectral series are without exception single atoms, not compounds of atoms.

We can in fact first place the beam of rays of moving positive atomic ions in a plane perpendicular to the axis in which we see the spectral lines emitted by them.

We have learnt through experience that when an electrical ray strikes the surface of an atom, an electron, and in some circumstances a second and even a third electron, can be detached.

The most common and most important result of them is that the nature and size of the effect on corresponding series of different elements are largely an expression of the peculiarity of their atomic structure - or, at least, of the structure of the surface.

The removal of an electron from the surface of an atom - that is, the ionization of the atom - means a fundamental structural change in its surface layer.