[17]While written in the third person the words are clearly his own.
[18]His determination of the Martian temperature has since been very closely verified.
[19]In a letter to Dr. V. M. Slipher on Oct. 4, 1902 he writes:
“There has come into my head a new way for detecting the spectral lines due to a planet’s own atmospheric absorption, and I beg you will apply it to Mars so soon as the Moon shall be in position to make a comparison spectrum.
“It is this. At quadrature of an exterior planet we are travelling toward that planet at the rate of 18.5 miles a second and we are carrying of course our own atmosphere with us. Our motion shortens all the wave-lengths sent us from the planet, including those which have suffered absorption in its atmosphere. When the waves reach our atmosphere those with a suitable wavelength are absorbed by it and these wave-lengths are unaffected by our motion since it is at rest as regards us. Even were the two atmospheres alike the absorbed wave-lengths reaching us would thus be different since the one set, the planet’s, have been shifted by our motion toward it while the other set, our own, are such as they would be at rest. We thus have a criterion for differentiating the two. And the difference should be perceptible in your photographs. For the shift of Jupiter’s lines due to rotation is such as 8. × 2. = 16 miles a second produces, which is less than 18.5 and about what you will get now.”
[20]So far as the shooting stars are concerned this opinion was based upon their velocities, which have since been found in many cases to be greater than was then supposed.
[21]Opic has recently shown that the sun’s effective domain is even larger.
[22]Later observations seem to show that Mercury’s periods of rotation and revolution are not the same, but nearly so.
[23]It now appears very improbable that these are real comet families.
[24]Recent results indicate that these are much smaller, and sometimes move faster, than was formerly believed.