ns of the object show on
the film. The sunlight, or the actinic rays in the sunlight, affect the
chemical material so that when the fixing chemical is applied it
prevents a change in the condition of the chemical."

"What do you mean by the actinic ray?"

"All light is vibration; the greatest motions which are perceptible to
the eye, being known as violet. Now there are still more rapid
vibrations than are put forth to make the violet rays, which are called
the actinic rays, and are the ones which affect the chemicals so
acutely."

"Is it then possible to photograph with a light that is not perceptible
to the eye?"

"You have heard of the x-rays, no doubt; they are the actinic or ultra
violet, which are above the visible light. These light vibrations are of
such a character that they penetrate many substances. A curious effect
of this was shown some time ago when a photograph was taken of the side
of a vessel which had several coats of paint over the old name, and the
photograph showed not only the new name, but also the old one beneath."

The time had now arrived when they must make preparations for the
proposed voyage of discovery around the island. It was a momentous time
for them. The boys could not help but look with longing eyes to the
cave. Before they went it was felt something more should be learned
about its mysteries.

The Professor was not at all backward in encouraging this feeling.

"Wouldn't it be a good thing to take such things out of the cave as we
can make use of here, and during our trip?" said Harry.

"What things do you think we could utilize?"

"Probably the guns; and then they have some cooking utensils."

"And why," suggested George, laughingly, "couldn't we take some of the
money along?"

"That would be a comfortable feeling to have plenty of money in our
pockets. Very well, we'll take this afternoon for the trip."

An early start was made, the lamps carefully trimmed and the guns,
together with the bolos, collected. It was a short walk to the opening,
and Angel, although not invited, accompanied the party.

Together they descended, and soon reached the scene of the conflict at
the large recess to the left of the entrance. The Professor, after
reviewing the scene, suggested that the bones should be carefully
gathered together and deposited at a place where they could be buried.

[Illustration: _Portugese Coin, 1700. Spanish Pistole Peruvian Dollar._]

[Illustration: _Fig. 15.