Next that 6 is a doublet of 13 sqq.
Wollaston's doublet (fig.
There is no Stokes' function when the axis of the doublet at S does not pass through 0; the image system will consist of an inclined doublet at H, making an equal angle with OS as the doublet S, and of a parallel negative line doublet, extending from H to 0, of moment varying as the distance from O.
A doublet is any class which possesses a member x such that the modified class formed by all the other members except x is a unit class.
The motion of these cylinders across the line of centres is the equivalent of a line doublet along each axis.
When S and S' lie on the same radius, taken along Ox, the Stokes' function can; be written down; and when S and S' coalesce a doublet is produced, with a doublet image at H.
Young, according to which the dark line observed in the centre of each component of the sodium doublet in a Bunsen burner is transparent to a radiation placed behind.