The number 8 refers to the magnification. In this case the object being viewed would appear to be eight times larger than seen with the naked eye. The number 40 refers to the diameter of the larger lens. The larger the number the bigger the lens. In this case it refers to 40 millimeters (about 1.5 inches). A large lens is desirable as it will let more light in and the thus the object being viewed will look brighter.