VII. Ocular spectres, structures and functions, mutual exponents

Our visual organs are not only capable, by an adjusting lenticular system, of painting, under varying conditions, images of luminous objects, upon a membrane in special relation with the brain, but involve many adjuvant structures; and thus it happens that they reveal to us a number of adventitious phenomena— spectres as we may call them, whether caused by light at the parts that cover the eyeballs, or within them, or by any stimulus whatever affecting the special nervous tract. These must be eliminated, if we would avoid the risk of ascribing effects begotten by subordinate parts to more integral portions of the apparatus.