The light of the body is the eye
The Vulgate Latin and all the eastern versions read "the light of thy body is thine eye". The sense is, that as the eye gives light, to the body, and the several members of it, by which they are guided and directed; so the understanding is the light of the soul, and the guide to all the powers and faculties of it;
therefore when thine eye is single, thy whole body also is full
as when the eye is free from vicious humours, and its sight is clear, the whole body reaps the advantage of it, and is perfectly illuminated by it; so when the eye of the understanding is opened and enlightened by the Spirit of God, into the truths of the Gospel, and a single regard is had unto them, and to the glory of Christ in them, the whole soul is filled with light, joy, and comfort:
but when thine eye is evil, thy body also is full of darkness;
as when the eye of the body is attended with any bad humours that hinder the sight, all the members of it are in darkness; so, when the understanding is darkened through the blindness and ignorance there is in men, with respect to Gospel truths, all the powers and faculties of the soul are in a very miserable and uncomfortable condition. The 35th and 36th verses are not in Beza's most ancient copy.