both an architectural feature and a method of introducing natural light into a space, and are typically wooden or metal structures with a number of glazed glass panels. Laylights As an element of architecture, a laylight is a glazed panel usually set flush with the ceiling for the purpose of admitting natural or artificial light. Laylights typically utilize stained glass or lenses in their glazing, but can also use alternative materials. For example, the Lyme Art Association Gallery utilizes translucent white muslin laylights below its skylights. A laylight differs from a glazed (or closed) skylight in that a skylight functions as a roof window or aperture, while a laylight is flush with the ceiling of an interior space. When paired with a roof lantern or skylight on a sloped roof, a laylight functions as an interior light diffuser. Before the advent of electric lighting, laylights allowed transmission of light between floors in larger buildings, and were not always paired with skylights. Atrium An atrium is a large open space located within a building. It is often used to light a central circulation or public area by daylight admitted through a glass roof or wall. Atria provide some daylight to adjacent working areas, but the amount is often small and does not penetrate very far. The main function of an atrium is to provide a visual experience and a degree of contact with the outside for people in the working areas. The daylighting of successive storeys of rooms adjoining an atrium is interdependent and requires a balanced approach. Light from the sky can easily penetrate the upper storeys but not the lower, which rely primarily on light reflected from internal surfaces of the atrium such as floor-reflected light. The upper stories need less window area than the lower ones, and if the atrium walls are light in color the upper walls will reflect light toward the lower stories. Translucent walls Walls made of glass brick are translucent-to-transparent. Traditionally they are hollow and grouted with a fine concrete grout, but some modern glass brick walls are solid cast glass grouted with a transparent glue. If the glue matches the refractive index of the glass, the wall can be fairly transparent. Increasing the amount of concrete, bottle walls embed bottles that run right through the wall, transmitting light. Concrete walls with glass prisms running through them have also been made. With the advent