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July 31, 1891.] THE PHOTOGRAPHIC NEWS. 539 by which we generally judge of nearer objects, are only known to sensation as such. In themselves, they are merely signs which must be mentally interpreted before we can apprehend distance. The accepted physiological theory fares no better than the mental one. It is well-known that distinct vision is obtained only when the part to be distinctly seen is centrally incident on the retina. Let anyone watch his own vision while reading, or, still better, watch the eyes of another reader as the direction of their axes moves along each line of the book under perusal, and he will under stand what is meant. Wundt uses the fact as a metaphor to illustrate the psychical doctrine of attention ; and Mr. Jas. Sully, in an article headed “ Unconscious Psychical Activity,”* says: “To adopt the metaphor of Wundt,the whole mental region (conscious and sub-conscious) answers to the total field of view present to the eye in varying degrees of distinctness at any moment when the organ is fixed in a certain direction. The latter region, that of attention or clear consciousness, corresponds to that narrow area of “perfect vision ” on which the glance is fixed. It would, perhaps, seem natural to suppose that these facts, relating as they do to our daily experience, should have been generally recognised, and be constantly present to our minds. But we are all accustomed, when gazing on a scene, to turn the eyes instantly on any part we desire to see, and then, having placed at least one clear vision of each part on the memory, it is held there by the faint sub-consciousness of it which sensation can produce when not looked at directly ; and, while it is imaged on an outer part of the retina, we are apt—all unaware of the really intricate complexity of these combined mental and physical processes, and perhaps thinking of it all simply as " sight -—-innocently to attribute the comparative clearness of the mental image—which, though partly pro duced by obscure sensation, is illumined by a more vivid memory—wholly to that present sensation, which is, never theless, quite insufficient by itself to produce it. It is thus difficult for us adequately to appreciate the real indistinctness of our visual sense of those parts of the general image which are away from the centre of the retina, and of which we cannot actually have—except as refreshed by at least one direct glance—any sense approaching the clear vision physically accompanying central definition, and the clear consciousness mentally accompanying the directed attention. Without at least one direct glance we can possess only an imperfect or sub-consciousness of them. For to the necessary optical indistinctness of oblique pencils, is here added the differ ence in sensitiveness between the outer parts of the retina, and of the central parts—especially of the fovea centralis, or small central pit—marked by the presence of fewer “cones” and more “rods.” To these again is added, in general experience, the effect of comparative inattention, blurring and confusing the mental impression. For our constant habit is always to accompany attention by so directing the eyes as that the point to which that attention is directed shall be imaged on or near the fovea centralis ; and as the attention is directed, so the axes of both eyes are converged, and also the ciliary accommodation is made. As Helmholtz says, “ Just as quickly as the eye turns, does the accommodation change,” and ‘ ‘ Whatever we want to see we look at, and see it accurately ; what we do not look at, we do not care for at * “ Outlines of Psychology," p. 74. the moment, and so do not notice how imperfectly we see it.”* In the light of these well accepted theories, it is difficult to treat seriously such a phrase as a “portion of the eye which perceives distance.” For, first, the per ception of distance is far more than the visual sensation of a distant object, and necessarily includes the mental interpretation which attributes distance to that object. Secondly, the signs which are thus interpreted have been obtained from many complex sources, including distinct sensation on the fovea centralis ; to which partit is, indeed, very difficult for us, even by direct effort, to avoid bringing the image of any object to which attention is even moment arily directed. For these reasons we may be quite sure that both the distant and near parts of the view have all been seen, and the distances all inferred, from vision or the memory of vision, when imaged on this fovea centralis. Different parts of the image cannot, therefore, definitely correspond to different parts of the ever-moving eye, as they may with different parts of a photographic lens, whose stony gaze is fixed and unvarying. It will be found that all the experiments recorded partake more or less of this same character—they are all judgments. For they are in no instance merely crude sensations, but perceptions which have been interpreted from sensations by mental action. It is in vain, there fore, to search for their origin among physical causes, such as imagined “ naturally selective action of the retinal nerves.” For if such action were even demontrated, we could thus only obtain signs to be mentally interpreted. We must turn to the mental rather than to the physical side of our nature, and then all may become comparatively clear. GLASS ETCHING. . ‘ 1 White acid ” is a name used by glass etchers to designate mixtures of hydrofluoric acid with various chemicals which are used for matting the surface of glass. The discovery of white acid is due to Berzelius, who, while engaged in his investigations on the properties of glass, made the discovery that fluoride of ammonia had the property of matting or opaquing glass. Since that time it has been found that other alkaline fluorides possessed the same power, and during the last few years this has been taken advantage of on a large scale for producing ornaments on glass of the greatest beauty. It is employed, principally, for producing ornamental figures on door lights, although it is used very extensively for decorating glass ware for table use, and also for the various sorts of globes used on lamps and gas fixtures. Extremely fine effects may be obtained on mirrors, and the silvering may be placed on either the same or the opposite side from the etching. During the last few years etching on glass has shown itself as a formidable rival to the sand blast, the work generally being indistinguishable from that produced by the latter, except that acid is capable of producing effects of a much greater fineness and delicacy. The grinding is much more even, and therefore more easily cleaned. In Germany, where the art has been carried to a much higher point of perfection than elsewhere, a number of formula! for matt-etching are in use. Within a short time some of these have been published in various scientific journals, but they all belong to the category of what might be called slow acids, and are very unreliable and • Helmholtz’s Popular Lectures, p. 214.
