Yijin Zhai is the name of the studio belonging to the Qianlong Emperor's eleventh son, Yong Xing (1752-1823), also known as Prince Cheng.
The inscription may be translated as: 'At Yijin [zhai], Zhoufeng [lou]. The two kinds of ink brushes (I have been presented with) are made of hair that has been carefully chosen and is of the finest quality. Neither too hard nor too soft, the texture is perfect. Do allow me to thank you in person. Last night, sitting alone, I pondered for a long time and twice adjusted the wick, yet failed to come up with a single word. You cannot imagine how indolent I have become. Prince Cheng.'