10. Crown glass, 1 millimetre thick, covered with a very thin layer of
gold.

11. Red crown glass, 2 millimetres thick.

12. Block of Iceland spar (very transparent to ordinary light, but
very opaque to Röntgen rays).

13. A bit of tinfoil.

14. Aluminium medal, showing faint traces of the design and lettering
on both sides, as if it were translucent.

15. Metallic mirror, shows no effect of regular reflection.

16. Bit of sheet-lead, 1 millimetre thick.

17. Quarter-of-a-dollar coin, silver.

18. Piece of thin ebonite, such as is used for photographic
plate-holder.]

[Illustration: DR. WILLIAM KONRAD RÖNTGEN, DISCOVERER OF THE X RAYS.

From a photograph by Hanfstaenge, Frankfort-on-the-Main.]