In his second course of lectures he treated of light as a means of investigation. One of the objects taken up was the nature of comets. He held that the nucleus consists, in its inner portions at least, of vapor of some kind in an incandescent state. To explain the cause of this incandescence he brings forward the "greenhouse theory." The glass of a greenhouse is transparent to the higher but opaque to the lower forms of radiation, and hence acts as a trap for the sun's rays. The nucleus of the comet he supposed to be surrounded by an envelope of some kind, transparent to the higher but opaque to the lower forms of radiation. Thus solar heat can get freely at the nucleus, but cannot escape until it has raised the nucleus, in part at least, to incandescence. The coma and tail are formed by the condensation of small quantities of this vapor, so that they are mere mists of excessive tenuity. Prof. Tait preferred his own "brickbat theory "; he considered that Stokes' theory made the comet resemble the huge but barely palpable Efreet of the Arabian Nights, who could condense himself so as to enter the bottle of brass with the seal of Solomon the son of David. (Nature, August 20, 1885.)
The third course of lectures treated of the beneficial effects of light. As regards the special application contemplated by Burnett, he concludes: "If we shut our eyes to the grandeur of Nature and do not attempt, through the things that are made, to acquire higher conceptions of the eternal power and Godhood of the Maker, our conceptions of the Divine Being are apt to become too anthropomorphic. If on the other hand we confine our attention to the study of Nature in all its immensity, our conceptions of its Author are in danger of merging in a sort of pantheistic abstraction in which the idea of personality is lost." Tait remarked with reference to these sentences that the first Burnett lectures had set a noble example to successors, and that Stokes had supplied a valuable warning not only to them but "to the rapidly changing quaternion of neo-teleologists that were soon to be set to work in the Scottish Universities." He referred to the new institution of the Gifford Lecturers.
