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 = _h_ν. The Balmer-Ritz formula for the frequencies of the lines in the hydrogen spectrum is ( 1 1 ) ν = K ( ------ - -------) ( _n″_² _n′_² ) [Illustration: DIAGRAMS OF SPECTRA AFTER BUNSEN AND KIRCHHOFF] [Illustration: STRUCTURE OF THE RADIUM ATOM] [Illustration: MAIN LINES OF THE ATOMIC STRUCTURE OF SOME ELEMENTS]