The lateral (spectral) images occur in such directions that the projection of the element (a+d) of the grating upon them is an exact multiple of X.
Lord Rayleigh,' who has also investigated vibrating systems giving series of lines approaching a definite limit of " root," remarks that by dynamical reasoning we are always led to equations giving the square of the period and not the period, while in the equation representing spectral series the simplest results are obtained for the first power of the period.
The different values of the angle of minimum deviation for rays of different refrangibilities give rise to spectral colours, the red being nearest the sun, while farther away the overlapping of the spectra forms a flaming colourless tail sometimes extending over as much as ro° to 20 °.
Thus a monochromatic image of the sun, formed of a great number of successive images of the spectral line employed, will be built up on the plate.
In order to bring a spectral line upon the camera slit, the slit is widely opened and the plane mirror (f) rotated until the line is seen.