PICTURES TAKEN BY PROFESSOR ARTHUR W. WRIGHT OF YALE COLLEGE, SHOWING THE DIFFERENCES BETWEEN SUBSTANCES IN PENETRABILITY TO THE RÖNTGEN RAYS.
1 and 3. Flint glass prism (very opaque).
2. Quartz prism, showing transmission of the rays through the thin edges.
4. Prism of heavy glass, more opaque than flint glass.
5. One-cent coin, copper.
6. Five-cent coin, nickel.
7. White-crown glass, 1-1/2 millimetres thick.
8. Blue crown glass, 2 millimetres thick.
9. Yellow crown glass, 1-1/2 millimetres thick.