[1] _Astronomy and Astrophysics_, iii. 752, and _Astron. Soc. of the Pacific, Publications_, vi. 273 and ix. 109.
[2] According to Percival Lowell these results were, however, inconclusive because the strong atmospheric lines lie redwards beyond the part of the spectrum then possible to observe. Subsequently, by experimenting with sensitizing dyes, Dr Slipher of the Lowell Observatory succeeded in 1908 in photographing the spectrum far into the red. Comparison spectrograms of Mars and the Moon, taken by him at equal altitudes on such plates, eight in all, show the "a" band, the great band of water-vapour was distinctly stronger in the spectrum of Mars, thus affording what appeared decisive evidence of water vapour in the atmosphere of the planet.
[3] Lowell, _Mars and its Canals_, p. 101.
[5] _Proc. Amer. Acad. Arts and Sciences_, vol. xlii. No. 25.
[6] Professor F. W. Very concurs with Lowell (_Phil. Mag._, 1908).
[7] According to Lowell, the climatic conditions are proportionally warm in summer.
[8] The great space penetration of the Lowell Observatory is shown in the case of stars. More stars have been mapped there in a given space than at the Lick, and Mr Ritchey of the Yerkes Observatory found stars easily visible there which were only just perceptible at Yerkes.
[9] As against this, Lowell's answer is that the effect is not optical; for the belt surrounds the _melting_, not the _making_ cap.
[10] For limits of this theory and Lowell's view of its inapplicability to Mars, see _Astrophys. Jour._, Sept. 1907.
[11] Prof. Lowell's theory is supported by so much evidence of different kinds that his own exposition should be read _in extenso_ in _Mars and its canals_ and _Mars as the abode of life_. In order, however, that his views may be adequately presented here, he has kindly supplied the following summary in his own words:--