The infra-red requires special appliances; it has been examined visually by the help of phosphorescent plates (Becquerel), and with special photographic plates (Abney); but the most efficient way is to use the bolometer or radiomicrometer; by this means some 500 or 600 lines have been mapped.
In addition, taking advantage of the accuracy with which the bolometer can determine the position of a source of heat by which it is affected, he mapped out in this infra-red spectrum over 700 dark lines or bands resembling the Fraunhofer lines of the visible spectrum, with a probable accuracy equal to that of refined astronomical observations.
Such a bolometer receiver has been used by C. Tissot (Comptes rendus, 1904, 137, p. 846) and others as a receiver in electric wave telegraphy.
For details of the coelostat applied to the Snow telescope - the most perfect installation for spectroheliograph and bolometer work yet erected - see The Study of Stellar Evolution by Prof. G.
There is, however, still room for improved determinations of the moon's heat by the use of the bolometer in its latest form.