Ewald Hering's (1879) "On Muscle Sounds of the Eye": A translation and commentary.

Investigations of eye movements were transformed by Ewald Hering in 1879. He developed a novel method for recording them using the muscular sounds attendant on their rapid movements. Brief "clapping" sounds could be heard with the aid of a device like a stethoscope placed on the eyelid and they occurred when afterimages or "floaters" were seen to move. Hering applied the technique to record eye movements during reading and he called the rapid eye movements Rucke (jerks in English). Hering published a long review of eye movements and spatial vision later in 1879, but without a description of the muscle sounds. Hering's insightful article has been overlooked and a translation of it into English is presented.

Revealing the Concealed: Alternatives to Random Dots for Stereograms.

Investigations of stereoscopic depth perception were transformed via the use of computer-generated random-dot stereograms in the 1960s. They realized Wheatstone's wish of demonstrating binocular depth without monocular object recognition, and they have been the dominant stimulus for studying stereopsis since then. Alternative carrier patterns to random dots, based on graphics, photographs, and their combinations, are presented as anaglyphs and for free fusion. A wider range of concealed patterns can be revealed with these alternatives, and presenting them as anaglyphs can yield patterns that have visual appeal independent of the depth they conceal.

Reversing the eyes and reverse perspectives: Pseudoscopic amplification of reverspectives.

Stereoscopic photographs of works in reverse perspective do not reveal their three-dimensional structure whereas pseudoscopic photographs enhance the apparent depth effects.

Mario Ponzo (1928) on perception of numerosity: A translation and commentary.

Ponzo is a familiar name in psychology because of the illusion that takes his name. He had a long and productive career in Italy, and some of his work was translated for international journals already in his lifetime. However, few of these papers are available in English. We provide a commentary that considers how his name came to be associated with an illusion he did not discover. We explain the content of several papers, some of which are often cited in a wrong context in the literature (i.e., papers on touch mentioned in relation to the Ponzo illusion). More importantly, we discuss his contribution to the study of perceived numerosity, and provide a full translation of his important 1928 paper, including a redrawing of its 28 illustrations.

Binocular portraiture.

Pictorial portraits are viewed with two eyes despite the fact that they are mostly monocular: they have been produced from a single viewpoint (either by painters or photographers). The differences between the images on each eye are a consequence of the separation between them rather than differences in two pictorial images. Viewing with two eyes detracts from the monocular cues to depth within the singular portrait because of information for the flatness of the pictorial surface. Binocular portraits, on the other hand, incorporate differences between two pictorial images producing perceptual effects that cannot be seen by a single eye alone. The differences can consist of small disparities that yield stereoscopic depth or large ones that produce binocular rivalry. Binocular portraits require viewing with a stereoscope, many varieties of which exist. Those shown here are anaglyphs which can be observed through red/cyan filters. They are not conventional stereoscopic portraits where the sitter is imaged from two slightly different locations. Rather, the binocular processes of cooperation (stereoscopic depth perception) and competition (binocular rivalry) are manipulated in the binocular portraits. The subjects shown in the anaglyphic portraits have been involved in the science and art of binocular vision.

Ernst Wilhelm Brücke on stereoscopic vision.

In the early 19th century the doctrine of identical retinal points, linked with the Vieth-Müller circle, was a pillar of German physiological optics. It was challenged by Wheatstone's observations of stereoscopic depth perception announced in 1838; he also advanced a cognitive theory of binocular vision that attacked physiological interpretations. In 1841 Brücke mounted a defense of the doctrine by questioning Wheatstone's observations and offering an alternative interpretation in terms of the integration over time of a rapid sequence of convergence eye movements. The theory could not be sustained because of evidence that stereoscopic depth occurred without eye movements. Brücke also questioned Wheatstone's observations that with some stereoscopic displays stimulation of identical retinal points could result in double vision. The binocular combination of circles differing in size was accounted for by differentially dissociating accommodation in opposite directions for each eye from convergence. Despite the negative reaction to Brücke's proposals, his speculations about the nature of rapid eye movements and of their neural basis were ahead of his time.

Alfred Wilhelm Volkmann on stereoscopic vision.

Wheatstone's demonstration of binocular single vision and depth with stimulation of non-corresponding points reverberated throughout mid-nineteenth century German visual science. It challenged the received view that single vision was a consequence of retinal correspondence otherwise objects were seen double. Wheatstone also argued that stimulation of corresponding points could yield double vision. He interpreted his experimental observations in psychological rather than physiological terms, as did Helmholtz later. Volkmann addressed both of these challenges in a long article on stereoscopic vision published in 1859. While he accepted the first of the questions Wheatstone posed Volkmann was more cautious with regard to the second. Volkmann was an experimentalist who applied psychophysical methods to determine thresholds for stereoscopic depth perception. In line with many of his colleagues in Germany, he took issue implicitly with Wheatstone's approach: how can the detailed quantitative experiments supporting Vieth and Müller's interpretation of binocular single vision be derailed by simple observations with a stereoscope? Unlike most of his colleagues, Volkmann was swayed in Wheatstone's favor through his own experiments: both physiological and psychological processes are involved in stereoscopic depth perception.

On the Art of Binocular Rivalry.

Binocular rivalry has a longer descriptive history than stereoscopic depth perception both of which were transformed by Wheatstone's invention of the stereoscope. Thereafter, artistic interest in binocular vision has been largely confined to stereopsis. A brief survey of research on binocular contour rivalry is followed by anaglyphic examples of its expression as art. Rivalling patterns can be photographs, graphics, and combinations of them. In addition, illustrations of binocular lustre and interactions between rivalry and stereopsis are presented, as are rivalling portraits of some pioneers of the science and art of binocular vision. The question of why a dynamic process like binocular rivalry has been neglected in visual art is addressed.

Helmholtz at 200.

Hermann von Helmholtz was born 200 years ago, but his influence on vision research is enduring. His legacy in vision is celebrated visually with anaglyphs that combine portraits of him with illustrations from his publications. Emphasis is directed principally to his Treatise on Physiological Optics. Among the optical instruments Helmholtz invented were the ophthalmoscope, ophthalmometer, and telestereoscope. Mention is made of his investigations into accommodation, colour vision, eye movements, stereoscopic vision, binocular rivalry, and lustre. Helmholtz also presented his analyses of vision and art in several of his Popular Lectures.

On Stereoscopic Art.

Pictorial art is typically viewed with two eyes, but it is not binocular in the sense that it requires two eyes to appreciate the art. Two-dimensional representational art works allude to depth that they do not contain, and a variety of stratagems is enlisted to convey the impression that surfaces on the picture plane are at different distances from the viewer. With the invention of the stereoscope by Wheatstone in the 1830s, it was possible to produce two pictures with defined horizontal disparities between them to create a novel impression of depth. Stereoscopy and photography were made public at about the same time and their marriage was soon cemented; most stereoscopic art is now photographic. Wheatstone sought to examine stereoscopic depth without monocular pictorial cues. He was unable to do this, but it was achieved a century later by Julesz with random-dot stereograms The early history of non-photographic stereoscopic art is described as well as reference to some contemporary works. Novel stereograms employing a wider variety of carrier patterns than random dots are presented as anaglyphs; they show modulations of pictorial surface depths as well as inclusions within a binocular picture.

The vision of Helmholtz.

Hermann Ludwig Ferdinand von Helmholtz (1821-1894) began investigating vision at a time when its study was undergoing a revolution. Laboratory experiments were augmenting the long history of naturalistic observations. Instruments of stimulus control enabled the manipulation of time and space in ways that had not been possible previously, and Helmholtz added to their tally. Vision was a central issue in his early years as an academic, and the bicentenary of his birth is here celebrated visually. Much of his research on vision was described in his Handbuch der physiologischen Optik, which was translated into English to mark the centenary of his birth. The history of his Handbuch is examined, together with illustrating highlights from it. Helmholtz's contributions to understanding the eye as an optical instrument, the sensations of vision, and perception were expressed in the three parts of the Handbuch, which became the three volumes of his Treatise on Physiological Optics.

On the Origins of Terms in Binocular Vision.

Vision with two eyes has been commented upon for many centuries, and the principal concern has been with binocular single vision. The terminology we apply to binocular vision developed rapidly after the invention of the stereoscope in the early 19th century. The origins of terms such as anaglyph, binocular lustre, chromatic stereoscope, cyclopean eye, dichoptic, horopter, pseudoscope, rivalry, stereoscope, stereograph, and stereopsis are described together with portraits of those who introduced them.

Albrecht Nagel on vision with two eyes.

Nagel's book on vision with two eyes was published in 1861, during a period in which German visual scientists were struggling to rescue the doctrine of identical retinal points from the evidence of stereoscopic depth. The long observational history of binocular vision has been dominated by the appearance of a single world with two eyes and its breakdown when the eyes are distorted abnormally. Early in the nineteenth century the flat horopter of Aguilonius (proposed two centuries earlier) assumed curvature in the form of the Vieth-Müller circle which was linked to identical retinal points: there were only two possible states of binocular perception - singleness with images on the Vieth-Müller circle and doubleness otherwise. This elegant edifice was undermined when Wheatstone demonstrated singleness and depth from images with slight retinal disparities. Nagel responded by providing observations on combining simple line stimuli in the two eyes. In the last part of chapter 3 of his book, Nagel describes experiments with lines varying in orientation or curvature with respect to the two eyes; it is in this section that Nagel draws attention to cyclofusion and the involvement of the extraocular muscles in it. Ocular torsion was an issue of considerable contention in nineteenth century visual science. The possibility of torsion in opposite directions seemed fanciful and yet this is what Nagel proposed in order to maintain cyclofusion for lines inclined in opposite directions relative to the horizontal. Similar rotations about the vertical resulted in a depth effect with no cyclovergence. The involvement of cyclovergence remained hotly debated until photographic recording of eye movements verified it.

Looking at Buswell's pictures.

In 1935 Guy Buswell, an educational psychologist at Chicago University, published How People Look at Pictures. In it he recorded photographically the eye movements of 200 observers when looking at a wide variety of pictures. He analysed the overall distribution of fixations on pictures, compared the first few fixations on a picture to the last few, measured the durations of fixations made early in viewing and those made near the end of viewing, examined how fixation duration changed with viewing time, recorded the consistency between different observers when viewing the same picture and he looked at the influence of instructions given to observers upon their eye movements when viewing a picture. He commented on the substantial differences between individuals and noted that instructions had a dramatic effect on the pattern of eye movements. Buswell's analysis was graphical rather than statistical. In this article Buswell's figures are recombined and his research is placed in the context of earlier investigations of eye movements with pictures by Stratton and Judd and later ones by Yarbus.

Ocular Equivocation: The Rivalry Between Wheatstone and Brewster.

Ocular equivocation was the term given by Brewster in 1844 to binocular contour rivalry seen with Wheatstone's stereoscope. The rivalries between Wheatstone and Brewster were personal as well as perceptual. In the 1830s, both Wheatstone and Brewster came to stereoscopic vision armed with their individual histories of research on vision. Brewster was an authority on physical optics and had devised the kaleidoscope; Wheatstone extended his research on audition to render acoustic patterns visible with his kaleidophone or phonic kaleidoscope. Both had written on subjective visual phenomena, a topic upon which they first clashed at the inaugural meeting of the British Association for the Advancement of Science in 1832 (the year Wheatstone made the first stereoscopes). Wheatstone published his account of the mirror stereoscope in 1838; Brewster's initial reception of it was glowing but he later questioned Wheatstone's priority. They both described investigations of binocular contour rivalry but their interpretations diverged. As was the case for stereoscopic vision, Wheatstone argued for central processing whereas Brewster's analysis was peripheral and based on visible direction. Brewster's lenticular stereoscope and binocular camera were described in 1849. They later clashed over Brewster's claim that the Chimenti drawings were made for a 16th-century stereoscope. The rivalry between Wheatstone and Brewster is illustrated with anaglyphs that can be viewed with red/cyan glasses and in Universal Freeview format; they include rivalling 'perceptual portraits' as well as examples of the stimuli used to study ocular equivocation.

The action of light on the retina: Translation and commentary of Holmgren (1866).

In 1866, Holmgren published an account of the physiological action of light on the retina. The article is taken as the origin of research on the electroretinogram, although the term was not introduced until much later. We present a translation of the article into English and provide a commentary on its reception and significance.

Microscopic anatomy of sensory receptors.

Experiences following stimulation of the senses have been recorded for millennia, and they could be related to the gross anatomy of the sense organs. Examination of their microanatomy was to await the development of achromatic microscopes in the early nineteenth century. Among the microscopic structures that were isolated and described were specialized sensory cells, called receptors, and they could be related to the stimuli that excited them. Those located in well-defined sense organs (like the eyes, ears, nose, and tongue) were named on the basis of their morphology, whereas the receptors in or beneath the surface of the skin were generally named after those who first described them. Illustrations of early representations of sensory receptors are combined with "perceptual portraits" of the microanatomists who described them.

Receptor Visionaries.

Sensory receptors were described and illustrated after they had been observed with the aid of microscopes. Most descriptions were made in the 19th century after the introduction of achromatic lenses in microscopes. In some senses (like vision), receptors were named according to their morphology whereas in others (like touch), they are known by the names of those who initially described them. Illustrations of the receptors from original sources are here combined with portraits of their originators.

Early Studies of Binocular and Binaural Directions.

Understanding how the eyes work together to determine the direction of objects provided the impetus for examining integration of signals from the ears to locate sounds. However, the advantages of having two eyes were recorded long before those for two ears were appreciated. In part, this reflects the marked differences in how we can compare perception with one or two organs. It is easier to close one eye and examine monocular vision than to "close" one ear and study monaural hearing. Moreover, we can move our eyes either in the same or in opposite directions, but humans have no equivalent means of moving the ears in unison. Studies of binocular single vision can be traced back over two thousand years and they were implicitly concerned with visual directions from each eye. The location of any point in visual or auditory space can be described by specifying its direction and distance, from the vantage point of an observer. From the late 18th century experiments indicated that binocular direction involved an eye movement component and experimental studies of binaural direction commenced slightly later. However, these early binocular and binaural experiments were not incorporated into theoretical accounts until almost a century later. The early history of research on visual direction with two eyes is contrasted to that on auditory direction with two ears.