How did the narrator let Ranga have glimpse of Ratna?

The narrator arranged the meeting very systematically. First, he called Ratna on the pretext of sending buttermilk through her. Then he asked her to sing a song. Meanwhile Ranga, whom he had sent for, reached the door. He became curious to see the singer and peeped in. His presence at the door blocked the light and Ratna stopped singing abruptly.