Just at this point of the ultraviolet region where the sun's spectrum becomes so interesting the spectrum terminates abruptly. We can produce shorter wave-lengths artificially but' we cannot detect them in sunlight, Moreover the spectra of all\the heavenly "bodies, including those of widely different types of stars, end\at this same wave-length. There can be only- one conclusion. Something in the earth's atmosphere, something quite invisible to us, absorbs these shorter wave-lengths so strongly that no trace of them has been definitely detected, Such a barrier is more surprising ¢|when it is realized that air at the earth's surface is transparent to short wave-lengths. A horizontal light path many miles in length, containing weight for weight much more air than one finds in the total path from the earth's surface to outer space, will still transmit ultra-violet light. It had been suspected that the absorbing something was ozone—common oxygen except that three atoms are linked together to form the molecule instead of the ordinary two—and in 1920 this was conclustvely demonstrated by two French investigators, Charles Fabry and H. Buisson.
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