I remember reading about a scientist who failed to discover x-rays (or
possibly some other form of radiation) in probably the late 1800s. 
His lab assistant told him that photographic film got clouded if
stored near a certain piece of equipment.  Instead of investigating
and discovering x-rays, the scientist told his assistant, "Well, don't
store the film there."

I thought this story appeared in Richard Rhodes' _Making of the Atomic
Bomb_, but an Amazon in-book search failed to find it.

Hello.

Well, you were absolutely right.  It is in Richard Rhodes' "Making of
the Atomic Bomb."

From Page 41 of that book:

"Thomson just missed discovering X-rays in 1894. He was not so unlucky
in legend as Oxford physicist Frederick Smith, who found that
photographic plates kept near the cathode-ray tube were liable to be
fogged and merely told his assistant  to move them to another place."
source: Page 41
The Making of the Atomic Bomb
by Richard Rhodes, found using "search inside" on Amazon.com 
http://www.amazon.com/exec/obidos/tg/detail/-/0684813785/

The same story appears in another book, "To Light Such a Candle:
Chapters in the History of Science and Technology":

"As so often happens with an important scientific discovery, others
had observed the effects previously but had failed to recognize it.
After Röntgen's discovery Sir William Crookes realized that years
before he had returned to the manufacturer some photographic plates
with the complaint that they were fogged; he later remembered that
they had been close to the discharge tube, and had been affected by
the X-rays emitted. Even more remarkable is the fact that the Revd
Frederick John Smith (1848-1911) had been warning people for some
years that photographic plates would be fogged unless they were kept
well clear of discharge tubes, even if they were well protected from
light; Smith, who later later changed his name to Jervis-Smith, was a
lecturer in experimental mechanics at Oxford."
Page 243
To Light Such a Candle: Chapters in the History of Science and Technology
by Keith J. Laidler
http://www.amazon.com/exec/obidos/tg/detail/-/0198500564/

---------

search strategy:
I initially thought that the scientist in question might have been
William Crookes because Wikipedia mentions him as someone who observed
X-rays' effect on photographic film but failed to investigate it.
See:
Wikipedia: X-rays
http://en.wikipedia.org/wiki/X-ray

By searching for "William Crookes" in books on Amazon, I found the
story in Laidler's book about Frederick Smith.  I then searched for
"Frederick Smith" in Rhodes' book.

I hope this helps.

Thank you for the tip.
-juggler