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Jean Theophile Desaguliers (1683-1744) and eighteenth century vision research.

Nicholas J. Wade [*]

The emergence of psychology as an empirical discipline was influenced to a great extent by experimental investigations of visual phenomena, particularly in the nineteenth century. Less attention has been paid to experimental enquiries conducted in the eighteenth century, especially those of Jean Th[acute{e}ophile Desaguliers (1683-1744). He was an ardent advocate of Newtonian optics, on which he lectured and gave demonstrations. His research on colour and binocularity is outlined, together with those of other students of vision in that century. Experiments on visual vertigo conducted at the end of the century are also described. In 1716 Desaguliers reported a method of binocular combination that became widely employed in other studies of binocular vision, namely, placing an aperture in such a position that two more distant, adjacent objects were in the optical axes of each eye. Under these circumstances red and green patches of silk did not mix after the manner of combining prismatic lights, but engaged in ri valry. Desaguliers also investigated size perception and showed that apparent size was determined by apparent distance rather than physical distance. Moreover, he did not base his conclusions on his own observation but on those of 'any unprejudic'd Person'. Thus, both stimulus control and the use of the unbiased observer were employed in eighteenth century experimental studies of vision.

All the Knowledge we have of Nature depends upon Facts; for without Observations and Experiments, our natural Philosophy would only be a Science of Terms and an unintelligible Jargon (Desaguliers, 1745, p. v).

The study of vision has been central to that of psychology, particularly in its formative years. The appreciation that vision involves a psychological dimension is at the heart of empiricist philosophy, and investigations of colour and spatial phenomena laid the foundations for an empirical discipline of psychology. The major impetus came in the early nineteenth century with the invention of instruments for stimulus control that enabled experimental investigations of the temporal and spatial aspects of vision (see Hatfield, 1990; Wade & Heller, 1997). These, together with establishing psychophysical methods, indicated that psychological phenomena were open to systematic experimental enquiry. However, experiments on vision were conducted in the eighteenth century, particularly in the context of colour and binocularity, and these have generally been neglected in historical surveys. The principal focus of this article is on Jean Th[acute{e}]ophile Desaguliers (1683-1744), who carried out experiments on colour an d binocularity early in the century, and also made speculations about the mechanism of accommodation. Towards the end of the century sophisticated experiments on visual vertigo were conducted and these are also mentioned.

Vision research in eighteenth century Britain was conducted in the context of either optics or medicine. Newton's Opticks was published in its first decade and Young's initial observations on vision in its last (Young, 1793). Newton and Young are contrasted with respect to their theories of light, although their studies of vision had much more in common. Newton (1704) made many astute comments about vision and his optics were extended further in the visual domain by Desaguliers (1716a, 1728), Smith (1738) and Harris (1775). The medical dimension was represented by Cheselden (1728) and the Darwins (Erasmus in 1794 and Robert in 1786). Porterfield (1759) and Wells (1792) combined optics and medicine with a flavouring of philosophy. Students of optics and medicine shared an interest in unravelling the enigma of accommodation (see Wade, 1998b). A topic that fused medical and philosophical issues was Cheselden's (1728) report of vision in a young man recovering from removal of a cataract. This became known as Mol yneux's Question, and it has stimulated considerable interest and speculation ever since (see von Senden, 1960; Morgan, 1977). Philosophers rarely questioned whether the person with sight restored would be able to see post-operatively, but only whether they could name objects by sight alone. Physicians, however, were faced with the practicalities of vision in those with sight restored. In the early nineteenth century the uniqueness of Cheselden's case became apparent because of the difficulties involved in making similar general statements from other cases (see Wade, 1998a).

The writings of Newton and Young are well known, and are not the principal concern of this article. Rather, the intention is to draw attention to the relatively neglected experiments of Jean Theophile Desaguliers (Fig. 1). He was often called John Theophilus because, although born in France, he was brought to England when only 2 years old; his father was a divine, fleeing religious persecution in France. He was educated by his father, and eventually assisted in teaching at his father's school in London. On the death of his father he studied natural philosophy at Christ Church, Oxford. He attended lectures on optics and mechanics that were delivered by John Keill, who had started lecturing on Newtonian natural philosophy around 1705. In 1710 Desaguliers was requested to deliver the lectures on experimental philosophy (physics), and he subsequently published his lecture notes in 1719. In the Preface he regretted having published them somewhat prematurely, but he attributed this to the actions of a student who had attended his lectures and had published and sold copies made from them, without Desaguliers's knowledge. His demonstrations of phenomena to students were a great innovation, about which there was much debate at the time, and his two textbooks based on his lectures (Desaguliers, 1719, 1744 and 1745) were very popular.

Desaguliers was an ardent advocate of Newtonian philosophy, and used it to disparage that of Descartes (1745, p. vi):

It is to Sir Isaac Newton's Application of Geometry to Philosophy, that we owe the routing of this Army of Goths and Vandals in the philosophical World[ldots] Our incomparable Philosopher has discovered and demonstrated to us the true Nature of Light and Colours, of which the most sagacious and inquisitive Naturalists were entirely ignorant.

At Newton's behest Desaguliers (1716a, 1716b, 1728) repeated some experiments in optics that Newton's detractors had failed to replicate; their confirmation by Desaguliers was such that 'no person, who chose to give his name to the public, or whose name is worth recording, made any more opposition to it' (Priestley, 1772, p. 351). These experiments were included in his first series of lectures and demonstrations on experimental philosophy (published in 1719), but not in the second series: 'As the Treatise of Opticks, I design'd to publish, was only intended to be easy and popular; I refer the Readers who are desirous of seeing the Subject treated of in that manner, to the Book of Opticks published by the Reverend and Learned Dr. Smith' (1744, p. vii). The lectures themselves were very popular and some were attended by royalty. Newton in turn was a supporter of Desaguliers's popularization of his theories. During the period of Newton's presidency of the Royal Society (1703-1727), Desaguliers was elected a Fel low in 1714, and became the Society's demonstrator and curator. He was awarded the Society's Copley Medal three times. His experiments in binocular vision and size perception are described below.

Binocular single vision

Throughout the eighteenth century binocular vision was studied in terms of singleness rather than depth (see Wade, 1987). In the context of experiments on binocular single vision, Desaguliers (1716 b) devised a method of combining different stimuli in the two eyes that was to become widely employed in other studies of binocular vision, namely, placing an aperture in such a position that two adjacent objects were in the optical axes of each eye (see Fig. 2). Desaguliers used the method to examine both binocular single vision and binocular colour combination, and to provide experimental evidence to support Newton's theory of binocular combination. The latter involved the concept of corresponding points and physiological union in the visual system. Newton's interest in the visual pathways, and in the ways in which messages from the two eyes could be combined, was stimulated by Briggs (1682), who sent his paper to Newton. Their correspondence indicates that Newton had reservations about Briggs's ideas (see Brewst er, 1855; Turnbull, 1960). In order to rise above the level of opinion, Newton carried out experiments on the dissected nerves of the visual pathways, he made the first representation of partial crossing of fibres at the optic chiasm, and he advanced a theory of binocular single vision based upon it (see Wade, 1987). Newton proposed that there were corresponding points in each eye and that singleness of vision resulted from their stimulation. While he demonstrated partial decussation of optic nerve fibres at the optic chiasm he erred regarding their union: fibres from corresponding points on the two retinas were represented as joining one another rather than remaining independent as far as the visual cortex.

The subtlety of Newton's analysis was not, however, widely disseminated. He did make passing reference to it in Query XV of his Opticks (1704), but it was not accompanied by a diagram, or by an account of the experiments he had performed. In presenting this as a query, rather than the report of an experiment (as in the unpublished manuscript, eventually printed by Harris in 1775), its speculative nature would have been reinforced. Desaguliers (1716b) restated Newton's definition of corresponding points and used his aperture experiment as a demonstration of its validity. With bifixation on the aperture a single candle is seen at B; with bifixation at the plane of the candles, two holes will be seen (Fig. 2).

The existence of retinal disparities was clearly enunciated by several students of optics in the eighteenth century, but the purpose to which they could be put--stereoscopic vision--was not appreciated. For example, Le Clerc (1712), an authority on perspective, represented the disparate projections of objects to the two eyes, but used these as evidence against Descartes' theory of binocular union in the pineal body. Both Smith (1738) and Harris (1775) provided clear diagrams of crossed and uncrossed disparities, but these were used to specify the locations of double images. The significance of retinal disparities for stereoscopic depth perception was even overlooked by Wells (1792), who conducted elegant experiments on binocular visual direction.

Binocular colour combination

The combination of different colours presented to corresponding regions of each retina became an issue of theoretical importance following Newton's experiments on colour mixing and his theory of binocular combination. Indeed, Desaguliers (1716b) was among the first to draw attention to the phenomenon. In particular, Desaguliers showed that dichoptically presented coloured lights rival rather than combine as in Newton's experiments on colour mixing. Using the same experimental apparatus as he employed for his studies of binocular single vision, he replaced the candles with patches of different coloured silks (Fig. 2) and observed that colour mixing did not occur. Moreover, if the coloured patches were made more intense, the rivalry was more compelling (p. 451):

But if instead of the Candles, [rho] be a piece of red Silk, and [gamma] a piece of green Silk, the same Position of the Eyes will make the Image at B, appearing like a red and green Spot together without a Mixture of Colours, If [rho] be a red hot Iron, and [gamma] a Candle of Sulphur, the Phaenomenon will he more distinct.

That is, no colour combination took place dichoptically, and the colour rivalry is more evident with intense stimuli. It is difficult to divine the extent to which this was based on observation or on a desire to support Newton's theory. Newton had stated that it was impossible for two objects to appear in the same place--because he believed that fibres from corresponding locations in each eye united at the chiasm. A similar argument would apply to colour, and Desaguliers' observation was certainly in line with Newton's prediction. Nonetheless, the report set in train a series of studies that attempted to examine the phenomenon.

Desaguliers' method was applied by Taylor (1738), who added the refinement of placing coloured glasses in front of candle flames; he found that colours combined rather than engaged in rivalry. Du Tour (1760) provided a clear description of binocular colour rivalry. He achieved dichoptic combination by another means: he placed a board between his eyes and attached blue and yellow fabric in equivalent positions on each side, or the fabric was placed in front of the fixation point. When he converged his eyes to look at them they did not mix but alternated in colour. Du Tour also applied the method of observing the colours through an aperture, as adopted by Desaguliers, and obtained similar results. Yet another technique was to view different coloured objects through two long tubes, one in each optic axis. This was used by Reid (1764), and he saw the colours combined although his description was not without its ambiguity: the colours were not only said to be combined, but also one 'spread over the other, withou t hiding it' (p. 326). In these early reports of binocular colour combination we have the origins of a dispute that was to extend beyond the eighteenth century, and was the source of much acrimony between two towering theoretical opponents in the second half of the nineteenth century-- Helmholtz and Hering: do colours fuse when presented to corresponding regions of each eye, or do they undergo rivalry?

Size perception

The knowledge that some objects were too small to be seen is an ancient one, but it was usually associated with their distance from the observer rather than their projected size. Measurement of visual acuity is a more recent concern. Hooke became interested in visual acuity because of its importance in making astronomical observations, and he was one of the first to measure it experimentally. In 1674 he gave a demonstration to Fellows of the Royal Society in order to determine whether they could distinguish a separation subtending less than one minute of arc; in general they could not (see Birch, 1757). Smith (1738) determined the limit of visual acuity to be about two thirds of a minute, which he calculated to subtend an 8000th of an inch on the retina. Porterfield (1738) assumed that the limits of visual acuity were based on the size of the retinal nerves themselves, and 'this Experiment of Dr. Hook's, serving to determine the minimum visibile, affords us a pretty certain Proof of the Magnitude of our nervo us Fibres' (p. 250). The existence of retinal receptors was not known at that time, and the retina was thought to be composed of the terminations of the optic nerve fibres. The value Porterfield derived for a minimum visible of one minute was a 7200th part of an inch.

Although visual resolution was dependent on visual angle, visual size was not. Statements about apparent size were rarely given empirical weight prior to experiments by Desaguliers, apart from the reports of the difference in the apparent size of the moon at the zenith and near the horizon--the moon illusion (see Hershenson, 1989). Desaguliers (1736a) compared judgments of the size of stimuli (candles) at different distances but matched for apparent size. When two candles of equal physical size were so perceived (even when one was twice the distance of the other), he substituted a smaller one of equal visual subtense for the far one, with no change in perceived size (see Fig. 3). He concluded that apparent distance, rather than physical distance, determines apparent size. It is noteworthy, too, that Desaguliers did not base his conclusions on his own observation but on those of 'any unprejudic'd Person'. Using na[ddot{i}]ve participants in perceptual studies was indeed a novelty, and one which was not adopte d by many others until the late nineteenth century.

Desaguliers' experiments on apparent size were stimulated by his speculations on the link between size and distance perception in the moon illusion. He pointed out, as had many others previously, that the horizon moon is perceived as more distant than at its zenith. The suggestion that the vault of the heavens was flat was made by Ibn al-Haytham (or Alhazen) in the eleventh century (see Plug & Ross, 1989), and Desaguliers (1736 a) expressed this concept diagrammatically (Fig. 4). This figure was modified by both Smith (1738) and Young (1807), and is usually associated with one of the latter. Desaguliers also considered aerial perspective (reduced contrast) as a cue to distance. The issue of size--distance invariance was not new at that time, but Desaguliers (1736 b) attempted to place it on an empirical footing with a simple experiment using two spherical balls. He noted that when the two spheres subtended equal angles the one of lower contrast was seen as more distant and larger.

Visual vertigo

Desaguliers restricted his analysis of vision to its optical features, as did most other students in the eighteenth century. One phenomenon did arise that drew attention to the involvement of involuntary eye movements in the perception of motion--visual vertigo. The occurrence of apparent motion both during and after body rotation has a long observational history (see Wade, 1998 a), but it was not subjected to experimental scrutiny until the late eighteenth century. It represents an interaction between the visual and vestibular systems, although the function of the latter was not appreciated until the early nineteenth century. Interest in visual vertigo was rekindled by Porterfield's (1759) contention that post-rotary apparent motion occurred despite the eyes remaining stationary; he assumed this to be the case because he had no feeling of the eyes moving following rotation. Robert Darwin (1786), the son of Erasmus and father of Charles, presented an alternative interpretation of postrotary visual motion; vis ual stimulation during rotation produced after-images that appeared to continue moving when the body was stationary. This was an area ripe for experiment as the methods of science could be applied without the need for any complex instruments. Wells (1792) was able to combine observation and experiment, in the way Desaguliers recommended, in order to demonstrate the interaction between eye movements and visual motion (see Wade, in press).

Vertigo is a disturbance of balance that was often referred to in the eighteenth century as dizziness or giddiness; it is usually accompanied by apparent visual motion of the surroundings. Erasmus Darwin (1794) devoted a chapter to it in his Zoonomia. The hypotheses of both Porterfield and Robert Darwin were roundly criticized by Wells (1792), who provided experimental evidence to refute them. The elegance of Wells's experiments lay not only in the description of the phenomenon, but also in the use of after-images as stabilized retinal images; those generated before body rotation could render visible the motions of the eyes after rotation ceased. Wells gave the first clear description of the fast and slow phases of post-rotary nystagmus, and its decreasing amplitude with time. Furthermore, he described how the direction of post-rotary after-image motion (and therefore eye movements) was dependent on head position during rotation. Like Porterfield, Wells was not aware of feeling any movements of the eyes afte r rotation and so he asked another person to rotate and then stop 'and I could plainly see, that, although he thought his eyes were fixed, they were in reality moving in their sockets, first toward one side, and then toward the other' (1792, p. 97).

Robert Darwin's (1786) interpretation of visual vertigo was given in an article on after-images; it was reprinted as the final chapter in the first volume of Zoonomia to which Erasmus added some more comments on vertigo, essentially supporting his son's theory against Wells's attack. Erasmus Darwin did not consider that eye movements were involved in post-rotary visual motion in part because dizziness could be experienced by a blind person. However, he did perform an insightful experiment on it, although he drew a false conclusion from the results. He rotated his upright body with the head tilted backwards to view a point on the ceiling over his head; when he stopped and looked horizontally objects appeared to rotate around the point of fixation. Darwin did not accept that the eyes could undergo torsion and so used this as evidence against any link between eye movements and apparent visual motion.

Following publication of Darwin's Zoonomia, Wells (1794a, 1794b) presented further experimental evidence correlating post-rotary apparent visual motion with eye movements. First, he demonstrated that visual vertigo occurs with rotation in darkness, contrary to the Darwins' speculation. Secondly, he carried out experiments indicating not only that the eyes moved following body rotation, but also how they moved. Together with these he provided a phenomenological description of the reducing amplitude of post-rotary nystagmus. Even more impressively, in the context of his dispute with Erasmus Darwin, Wells (1794b) demonstrated that the direction of nystagmus is dependent on the axis of the head during rotation, and that torsional nystagmus followed rotating the upright body with the head tilted backwards to view the ceiling.

Porterfield introduced the problem of the relationship between eye movements and visual vertigo and Wells essentially solved it by the subtle use of after-images as stabilized retinal images. Wells presented the first description of post-rotary nystagmus, distinguished between its slow and fast phases, noted its decline in amplitude over time, indicated how it can be suppressed, related the direction of nystagmus to head orientation, and established torsional nystagmus. Bell (1803) echoed Desaguliers' sentiment, given at the head of this article, when he surveyed this controversy: concluding in Wells's favour, he remarked: 'How superior is simple experiment to the most ingenious speculation!' (p. 293).

Conclusion

Desaguliers adopted Newton's natural philosophy and he applied experimental methods to the psychological domain of binocular vision and size perception. Not only did he apply control to the stimulus and viewing conditions, but he also tested naive observers. The involvement of larger samples of observers probably derived from his lectures and demonstrations which were both novel and very popular. He clearly delighted in his demonstrations, and in the year of his death he wrote (1745, p. x):

About the Year 1713, I came to settle at London, where I have with great Pleasure seen the Newtonian Philosophy so generally received among Persons of all Ranks and Professions, and even the Ladies, by the Help of Experiments; [ldots] the present Course, which I am now engag'd in, being the 121st since I began at Hart-Hall in Oxford, in the Year 1710.

Desaguliers' commitment to observation and experiment was implemented by Wells in his studies of visual vertigo.

Acknowledgements

The author is grateful to the reviewers and to the editor for their constructive comments on the first version of this manuscript.

(*.) Requests for reprints should be addressed to Professor Nick Wade, Department of Psychology, University of Dundee, Dundee DD1 4HN, UK (e-mail: n.j.wade@dundee.ac.uk).

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