conventional. 205
Sec. 5. Nature, and essential qualities of the open blue. 206
Sec. 6. Its connection with clouds. 207
Sec. 7. Its exceeding depth. 207
Sec. 8. These qualities are especially given by modern masters. 207
Sec. 9. And by Claude. 208
Sec. 10. Total absence of them in Poussin. Physical errors in his
general treatment of open sky. 208
Sec. 11. Errors of Cuyp in graduation of color. 209
Sec. 12. The exceeding value of the skies of the early Italian and
Dutch schools. Their qualities are unattainable in modern
times. 210
Sec. 13. Phenomena of visible sunbeams. Their nature and cause. 211
Sec. 14. They are only illuminated mist, and cannot appear when the sky
is free from vapor, nor when it is without clouds. 211
Sec. 15. Erroneous tendency in the representation of such phenomena by
the old masters. 212
Sec. 16. The ray which appears in the dazzled eye should not be
represented. 213
Sec. 17. The practice of Turner. His keen perception of the more
delicate phenomena of rays. 213
Sec. 18. The total absence of any evidence of such perception in the
works of the old masters. 213
Sec. 19. Truth of the skies of modern drawings. 214
Sec. 20. Recapitulation. The best skies of the ancients are, in
_quality_, inimitable, but in rendering of various truth,
childish. 215

CHAPTER II.--Of Truth of Clouds:--First, of the Region of the Cirrus.

Sec. 1. Difficulty of ascertaining wherein the truth of clouds
consists. 216
Sec. 2. Variation of their character at different elevations. The
three regions to which they may conveniently be considered
as belonging. 216
Sec. 3. Extent of the upper region. 217
Sec. 4. T