h unfinished. If you examine the
larger white fishing-boat sail, you will find it has a little spark
of pure white in its right-hand upper corner, about as large as a
minute pin's head, and that all the surface of the sail is gradated
to that focus. Try to copy this sail once or twice, and you will
begin to understand Turner's work. Similarly, the wing of the Cupid
in Correggio's large picture in the National Gallery is focused to
two little grains of white at the top of it. The points of light on
the white flower in the wreath round the head of the dancing
child-faun, in Titian's Bacchus and Ariadne, exemplify the same
thing.

[16] I shall not henceforward number the exercises recommended; as
they are distinguished only by increasing difficulty of subject, not
by difference of method.

[17] If you understand the principle of the stereoscope you will
know why; if not, it does not matter; trust me for the truth of the
statement, as I cannot explain the principle without diagrams and
much loss of time. See, however, Note 1, in Appendix I.

[18] The plates marked with a star are peculiarly desirable. See
note at the end of Appendix I. The letters mean as follows:--

_a_ stands for architecture, including distant grouping of towns,
cottages, etc.
_c_ clouds, including mist and aerial effects.
_f_ foliage.
_g_ ground, including low hills, when not rocky.
_l_ effects of light.
_m_ mountains, or bold rocky ground.
_p_ power of general arrangement and effect.
_q_ quiet water.
_r_ running or rough water; or rivers, even if calm, when their
line of flow is beautifully marked.

_From the England Series._

_a c f r._ Arundel. _a f p._ Lancaster.
_a f l._ Ashby de la Zouche. _c l m r._ Lancaster Sands.*
_a l q r._ Barnard Castle.* _a g f._ Launceston.*
_f m r._ Bolton Abbey. _c f l r._ Leicester Abbey.
_f g r._ Buckfastleigh.* _f r._ Ludlow.
_a l p._ Caernarvon. _a f l._ Margate.
_c l q._ Castle Upnor. _a l q._ Orford.
_a f l._ Colchester. _c p._ Plymouth.
_l q._ Cowes. _f._ Powis Castle.
_c f p._ Dartmouth Cove.* _l m q._ Prudhoe Castle.
_c l q._ Flint Castle.* _f l m r._ Chain Bridge over