I did not think; I investigated.
As quoted in "The New Marvel in Photography", by H. J .W. Dam, in McClure's magazine, Vol. 6, No. 5 (April 1896), p. 416, from Project Gutenberg
Röntgen has familiarized us with an order of vibrations of extreme minuteness compared with the smallest waves with which we have hitherto been acquainted, and of dimensions comparable with the distances between the centers of the atoms of which the material universe is built up; and there is no reason to suppose that we have here reached the limit of frequency.Wilhelm Röntgen