Numerous measurements have been made of the **wavelengths** of the brightest.

A body which absorbs all radiations of all **wavelengths** would be called a "perfectly black body."

If a collective system be corrected for the axis point for a definite wave-length, then, on account of the greater dispersion in the negative components - the flint glasses; - over-correction will arise for the shorter **wavelengths** (this being the error of the negative components), and under-correction for the longer wave-lengths (the error of crown glass lenses preponderating in the red).

The enclosed area for each temperature represents the total emission of energy for that temperature, the abscissae are the **wavelengths**, and the ordinates the corresponding intensities of emission for that wave-length.

In another direction, making a small angle with the last, such that the projection of AB upon it amounts to a few **wavelengths**, it is easy to see that the mode of interference is the same as if there were no ruling.

We may therefore put y=a sin -T (x - e) +b sin (x - f)+c sin 6 (x - g) +&c. (21) where the terms may be infinite in number, but always have **wavelengths** submultiples of the original or fundamental wave-length A.

Spectroscopic Measurements and Standards of Wave-Length.- All spectroscopic measurement should be reduced to **wavelengths** or wave-frequencies, by a process of interpolation between lines the wave-lengths of which are known with sufficient accuracy.