544) that an intelligible theory can be given which leads to the form j(OX) = c i /{exp(c 2 /**A9**) - I }, a form which agrees in a satisfactory way with all the experi ments.

The jet tube, for a reason which will be given when we consider the maintenance of vibrations, must be less than c **a9** half the length of the sounding tube.

It will be seen that the maximum ordinates lie upon the curve **A9** = constant dotted in the figure, and so, as the temperature of the ideal body rises, the wave-length of most intense radiation shifts from the infra-red X towards the luminous part of the spectrum.

Let a be the radius of a circle, and 0 (circular measure) the unknown angle subtended by an arc. Then, if we divide 0 into m equal parts, and L 1 denotes the sum of the corresponding chords, so that L i =2ma sin (0/2m), the true length of the arc is L1 +**a9** 3 - 5 + ..., where cp. =B/2m.