This picture must show be the beautiful Dominican tradition of the singing of the the Salve Regina and final blessing before processing to the dormitory.
The history of which is related as such:
Dominicans everywhere share the tradition of singing the Salve Regina and receiving a blessing with holy water at the words before they go to bed. Mother Francis Raphael Drane wrote that, “Dominic never had cell or bed of his own and slept, when he slept at all, in the church or the dormitory. One night, Dominic having remained in the church to pray, left it at the hour of midnight, and entered the corridor where the cells of the brethren were. When he had finished what he had come to do, he again began to pray at one end of the dormitory, and looking by chance towards the other end, he saw three ladies coming along, of whom the one in the middle appeared the most beautiful and venerable. One of her companions carried a magnificent vessel of water, and the other a sprinkler, which she presented to her mistress, and she sprinkled the brethren, and made over them the sign of the cross. But when she had come to one of the friars, she passed him over without blessing him; and Dominic having observed who this one was, went before the lady, who was in the middle of the dormitory, near to where the lamp was hanging. He fell at her feet, and though he had already recognized her, yet he besought her to tell him who she was. At that time the beautiful and devout anthem of the Salve Regina was not sung in the convents of the friars or of the sisters at Rome; it was only recited kneeling after Compline. The lady who had given the blessing said therefore to Dominic, ‘I am she whom you invoke every evening, and when you say, ‘Eia ergo advocota nostra,’ I prostrate before my Son for the preservation of this order. Then the blessed Dominic inquired who were the two young maidens who accompanied her, and she replied, ‘One is Cecilia, and the other Catherine.’ And the blessed Dominic asked again why she had passed over one of the brethren without blessing him; and he was answered, ‘Because he was not in a fitting posture;’ and so having finished her round and sprinkled the rest of the brethren, she disappeared.”
Drane, Augusta Theodosia (Mother Francis Raphael), The Life of St. Dominic and a Sketch of the Dominican Order with an Introduction to the America edition by Rev Joseph Sadoc Alemany, D.D., P. O’Shea Publisher, New York, New York, 1867, (pp. 136-138).