The primary importance and level of existence of chakras is posited to be in the psyche. However, there are those who believe that chakras have a physical manifestation as well.[27] The author Gary Osborn, for instance, has described the chakras as metaphysical counterparts to the endocrine glands,[28] while Anodea Judith noted a marked similarity between the positions of the two and the roles described for each.[29] Stephen Sturgess also links the lower six chakras to specific nerve plexuses along the spinal cord as well as glands.[30] C.W. Leadbeater associated the Ajna chakra with the pineal gland,[31] which is a part of the endocrine system. Edgar Cayce said that the seven churches of the Book of Revelation are endocrine glands.[32] However, these associations have never been scientifically verified. A development in Western practices dating back to the 1940s is to associate each one of the seven chakras to a given colour and a corresponding crystal. For example, the chakra in the forehead is associated with the colour purple, so to try and cure a headache a person might apply a purple stone to the forehead. This idea has proven highly popular and has been integrated by all but a few practitioners. Mercier introduces the relation of colour energy to the science of the light spectrum: As humans, we exist within the 49th Octave of Vibration of the electromagnetic light spectrum. Below this range are barely visible radiant heat, then invisible infrared, television and radiowaves, sound and brain waves; above it is barely visible ultraviolet, then the invisible frequencies of chemicals and perfumes, followed by x-rays, gamma rays, radium rays and unknown cosmic rays.[33] Understanding existence and physical form as an interpretation of light energy through the physical eyes will open up greater potential to explore the energetic boundaries of color, form and light that are perceived as immediate reality. Indian Yogic teachings assign to the seven major chakras specific qualities, such as color of influence (from the 7 rays of spectrum light), elements (such as earth, air, water & ether), body sense (such as touch, taste, and smell), and relation to an endocrine gland.[34] There are seven major chakras, which are arranged vertically along the axial channel (sushumna nadi). David Gordon White traces the modern popularity of the seven chakra system to Arthur Avalon's The Serpent Power, which was Avalon's translation of a late work, the Sat-Cakra-Nirupana.[9] Below is a description of the seven chakras, with various associations. Each of these chakras also have its elemental deity (Vasu) - demigod of its material element. From the top down they are: