The sight glass refers to the device used to observe the materials inside the container and equipment. It is a safety accessory and should be checked regularly. According to the shape, it is divided in o circular and rectangular mirrors. According to the purpose, it can be divided in o peeping and lighting mirrors. According to the structure, it is divided in o mirrors with lining and without lining, with neck, with cover, double-layer safety, heat preservation, with scraper, with washing device and mirrors suitable for powdery materials.