They didn't wish to attract too greatly the attention of the islanders.

Gradually, as their stay on the island went on, they learned the truth that Tu-Kila-Kila's eyes, as he himself had boasted, were literally everywhere.

For he had spies of his own, told off in every direction, who dogged the steps of his victims unseen.

Sometimes, as Felix and Muriel walked unsuspecting through the jungle paths, closely followed by their Shadows, a stealthy brown figure, crouched low to the ground, would cross the road for a moment behind them, and disappear again noiselessly into the dense mass of underbrush.
Then Mali or Toko, turning round, all hushed, with a terrified look, would murmur low to themselves, or to one another, "There goes one of the Eyes of Tu-Kila-Kila!" It was only by slow degrees that this system of espionage grew clear to the strangers; but as soon as they had learned its reality and ubiquity, they felt at once how undesirable it would be for them to excite the terrible man-god's jealousy and suspicion by being observed too often in close personal intercourse with their fellow-exile and victim, the Frenchman.

It was this that made them have recourse to the device of the heliograph.
So three or four days passed before Muriel dared to approach M.Peyron's cottage.