midnight
`` I wonder if it is the doctor walking in his sleep ? '' thought Billy . `` That would be a great joke , would n't it ? '' He thought he saw a flash of light for a moment , but was not sure of this , and hurried on after the midnight prowler , having just time to see him enter the window of the doctor 's cottage . `` Can it be the doctor after all ? '' he muttered . `` That would be funny after all .