IT is well known that the spectrum of the upper layers of the solar chromosphere is chiefly composed of those lines which are relatively more strengthened in the spark than in the arc, and which Sir Norman Lockyer originally styled enhanced lines. The best-known examples are the calcium H and K and the strontium pair (4216, 4077). According to modern theories of spectral emission, these lines are due to an atom which has lost one electron. The principal line due to the normal atom of calcium is the g-line 4227, and the corresponding Sr line is 4607, both of which occur at much lower levels. According to modern theories, therefore, Ca, Sr, and Ba atoms are more and more ionised as we approach the upper layers of the solar atmosphere, while in the lower layers both normal and ionised atoms occur.