“Too bad, it was a cloudy day. You really can’t see any
shadows.” My friend, the inventor Dennis Purcell, corrected me:
“I don’t think it was cloudy. It was a bright, sunny day. Or
perhaps cloudy-bright.” Dennis explained that most 19th century
photographic emulsions are blue-sensitive and hence cannot
record the sky – overcast, partially cloudy and sunny skies are
all overexposed. The sky is a featureless white, but the
“whiteness” of the sky is unrelated to the question of whether
there are clouds or whether you can see shadows. It was only
much later that panchromatic film was developed. (This accounts
for what I would call The Wisconsin Death-Trip Effect, after the
book by the same name. Scandinavian immigrants in turn-of-the-
century Wisconsin might not be insane, but they look insane
because their blue eyes are white in the pre-panchromatic
emulsions used to produce the photographs.) In both ON and OFF
the sky may look overcast, but you can see shadows. You have to
look at the shadows on the cannonballs