In this table n is the refractive index of the glass for sodium light (the D line of the solar spectrum), while the letters C, F and G' refer to lines in the **hydrogen spectrum** by which dispersion is now generally specified.

The most complete **hydrogen spectrum** is that measured by Evershed 8 in the flash spectrum observed during a total solar eclipse, and contains thirty-one lines, all of which agree with considerable accuracy with the formula, if the frequency number n is calculated correctly by reducing the wave-length to vacuo.9 It is a characteristic of Balmer's formula that the frequency approaches a definite limit as s is increased, and it was soon discovered that in several other spectra besides hydrogen, series of lines could be found, which gradually come nearer and nearer to each other as they become fainter, and approach a definite limit.

If we compare Balmer's formula with the general equation of Ritz, we find that the two can be made to agree if the ordinary **hydrogen spectrum** is that of the side branch series and the constants a', b, c and d are all put equal to zero.'