Below is given an explanation of the different symbols which occur at various places in the book; also the values of important physical constants.

1 m. = 1 metre; 1 cm. = 1 centimetre = 0·394 inches.

1 μ = 1 micron = 1/1000 of a millimetre = 0·0001 cm. = 10⁻⁴ cm.

1 μμ = 1/1,000,000 of a millimetre = 10⁻⁷ cm.

1 cm.³ = 1 cubic centimetre.

1 g. = 1 gram; 1 kg. = 1 kilogram = 2·2 pounds.

1 kgm. = 1 kilogrammetre (the work or the energy required to lift 1 kg. 1 m.).

1 erg = 1·02 × 10⁻⁸ kgm. = 7·48 × 10⁻⁸ foot-pounds.

λ represents wave-length.

ν represents frequency (number of oscillations in 1 second).

ω represents frequency of rotation (number of rotations in 1 second).

n represents an integer (particularly the Bohr quantum numbers).

The velocity of light is c = 3 × 10¹⁰ cm. per second = 9·9 × 10⁸ feet per second.

The wave-length of yellow sodium light is 0·589 μ = 589 μμ = 2·32 × 10⁻⁵ inches.

The frequency of yellow sodium light is 526 × 10¹² vibrations per second.

The number of molecules per cm.³ at 0° C. and atmospheric pressure is about 27 × 10¹⁸.

The number of hydrogen atoms in 1 g. is about 6·10²³.

The mass of a hydrogen atom is 1·65 × 10⁻²⁴ g.

The elementary quantum of electricity is 4·77 × 10⁻¹⁰ “electrostatic units.”

The negative electric charge of an electron is 1 elementary quantum (1 negative charge).

The positive electric charge of a hydrogen nucleus is 1 elementary quantum (1 positive charge).

The mass of an electron is ¹/₁₈₃₅ that of the hydrogen atom.

The diameter of an electron is estimated to be about 3 × 10⁻¹³ cm.

The diameter of the atomic nucleus is of the order of magnitude 10⁻¹³ to 10⁻¹² cm.

The diameter of a hydrogen atom in the normal state (the diameter of the first stationary orbit in Bohr’s model) is 1·056 × 10⁻⁸ cm.

The Balmer constant K = 3·29 × 10¹⁵.

The Planck constant h = 6·54 × 10⁻²⁷.

An energy quantum is E = hν.

The Balmer-Ritz formula for the frequencies of the lines in the hydrogen spectrum is

ν = K		1	-	1
		n″ ²		n′ ²