PART I-HISTORICAL

Researches in the field of indirect vision may be roughly divided into three classes,-those pertaining to the sensitivity of the peripheral retina to color, those pertaining to its sensitivity to light, and those pertaining to its sensitivity to forms, or its acuity for distinguishing objects. Varying numbers of studies have been made in each of these fields. The color sense has been most assiduously explored, the most painstaking and also the most recent extensive study being that of John Wallace Baird.' Less has been done in the other two fields but they too have not been neglected. Mention need be made at this point only of the work of Kirschmann, Wertheim,3 and Wirth."

The first published account of an attempt to explore the retina with a view to determine the form and extent of its sensitive surface we owe to Dr. Thomas Young. In a lecture before the Royal Society of London, in 1800, he said:

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The visual axis being fixed in any direction, I can at the same time see a luminous object placed laterally at a considerable distance from it; but in various directions the angle is very different. Upwards it extends to 50 degrees, inwards to 60, downwards to 70, and outwards to 90 degrees. These internal limits of the field of view nearly correspond with the external limit formed by the different parts of the face, when the eye is directed forwards and slightly downwards, which is its most natural position . . . . The whole extent of perfect vision is little more than 10 degrees; or more strictly speaking, the imperfection begins within a degree or two of the visual axis, and at the distance of 5 or 6 degrees becomes nearly stationary, until, at a still greater distance, vision is wholly extinguished. The imperfection is partly owing to the unavoidable aberration of oblique rays, but principally to the insensibility of the retina . . . The motion of the eye has a range of about 55 degrees in every direciton, so that the field of perfect vision, in succession, is by this motion extended to 110 degrees (pp. 44-46).

The Color Sensitivity of the Peripheral Retina, Washington, 1905. The reader is referred to this study for a summary of the literature in this field and for a bibliography.

2 August Kirschmann, Ueber die Helligkeitsempfindung im indirectem Sehen,' Phil. Stud., V, pp. 447-497.

3 Th. Wertheim, Ueber die indirecte Sehschärfe,' Ztsch. f. Psych. u. Physiol., pp. 172-187.

Wm. Wirth: 'Die Klarheitsgrade der Regionen des Sehfelds bei verschiedenen Verteilungen der Aufmerksamkeit,' Phil. Studien, June, 1906.

Philosophical Transactions, V, 91, 1801, p. 23ff.

Young does not indicate in detail how he obtained these results, but he experimented roughly with objects and 'luminous points.'

Purkinje' studied the acuity of indirect vision by placing knitting needles into a graduated paste-board arc, in whose center the eye was situated, and observing the distinctness with which they appeared when he fixated one of them.

The first actual measurements of retinal acuity are given by Hueck. He made a comparative study of the acuity of direct and indirect vision. The latter he investigated by means of black dots, lines, and squares of various dimensions on a white background, determining how far from the fixation point and with what size retinal image they could still be distinguished or recognized. He gives his results in a number of tables, the essentials of one of which are here reproduced. He found that lines fused according to the following data:

Volkmann introduced the method of momentary illumination with the electric spark, in order to eliminate the disturbing influence of eye movements. He endeavored to determine the sensitivity of the peripheral retina in several ways. First he determined at what distance from the eye, at various angles from the center, round black dots on white paper could still be recognized. He calculated the retinal images and compared them with the smallest image that could be perceived at the center. He repeated the experiment with an upright thread of a spider web and obtained angles only one-tenth as large as those from the dots. The only conclusion

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Johannes Evangelista Purkinje, Beitrage zur Physiologie der Sinne, 1825. For the data from Purkinje I am indebted to Aubert, Physiologie der Netzhaut, Breslau, 1865, pp. 336-337. The original sources were not accessible to me at the time of writing.

2 Mueller's Archiv., 1840, pp. 81-97.

Handwörterbuch der Physiologie (Wagner's), III, 1, 1846, p. 332f.

he draws at this point is that the acuity decreases rapidly from the center.

Volkmann next ascertained the smallest retinal image with which two parallel lines could be distinguished on various parts of the retina. The experiment duplicated the work of Hueck, but he obtained visual angles about ten times as large as those of Hueck. Volkmann noticed this discrepancy but made no attempt to explain it. The results of later investigators show Hueck to have done the more careful work.

Aubert and Foerster set themselves a problem in some respects similar to mine.' They determined both the extent and the shape of the field of vision within which certain units could be recognized. Their experiment they varied in a number of ways and they obtained some significant results.

In the first experiment to be noted they eliminated the influence of eye movements by illuminating the objects to be recognized with an electric spark. As such objects they used numbers and letters printed on paper charts 2 feet wide and 5 feet long. These charts they mounted on horizontal rollers so that the field could be changed between every two exposures. This prevented the subjected from becoming familiar with any part of it. They used four charts with characters of different sizes and at various distances apart; and they employed ten distances varying from .1 to 1 meter between the subject and the chart. They found that "the larger the angle made by the line of direction of an object with the line of sight, the larger must the visual angle of the object be in order to be recognized," (p. 239); and that "with a constant visual angle of the characters, small near characters can be recognized on a larger area of the retina than large distinct characters" (p. 240).

The latter peculiarity attracted Aubert's attention and he investigated it further, this time with constant illumination. Various distances from the fixation point were again used, a definite point was fixated, and the objects to be recognized were shifted in and out in various planes. The apparatus, by means of which definite positions of the head and objects were secured, need not here be described.2 As objects, two black squares upon white paper were used. Their distances apart were the same as their lengths of side, which were 20 mm., 8 mm., and 4 mm. These dimensions gave the same visual angle, 1° 8', at the distances of 1000, 400, and 200 mm. respectively.

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Physiologie der Netzhaut, pp. 237-253.

For a description and cut the reader is referred to p. 241, Op. Cit.

In the horizontal meridian the average of four observations gave the following angular extents of the visual fields:

For 20 mm. squares, distant 1000 mm., 39°

When the visual angle was changed to 34′ the results were:

For 8 mm. squares, distant 800 mm., 35°

More tables are given by Aubert, but it is sufficient to say that the data agreed closely with those obtained by the method of momentary illumination. With the same visual angle, small near objects may be recognized in a larger extent of the retina than large far objects; and for the same object a fairly constant ratio exists between the size of the visual angle of the object and its deviation from the fixation point. On the other hand, when the distance of the object is kept constant, the deviation at which the object may be recognized increases more slowly than the visual angle of the object.

To explain this puzzling phenomenon, as he calls it, Aubert assumed that in accommodation for distinct vision occurred a distortion of the rod and cone layer which disturbed the passage of the rays of light. The assumption that accommodation for the peripheral regions of the retina is less perfect in distant than in near vision he rejects. The shifting of the node, owing to changes in accommodation, does not explain the phenomenon either, for that should give just the opposite result.

In the experiment with momentary illumination both Foerster and Aubert noticed that more characters could be seen in the horizontal than in the vertical direction. This led them to a more careful exploration of the various meridians in order to ascertain how the sensitivity of the retina decreased. They did this again with constant illumination. They used a flat surface, the center of which was fixated, and they proceeded much as is frequently done in mapping the color zones. They recognized that they might have used an arc, but as they explored only a small field near the fixation point, they regarded this unnecessary. As objects, they used this time round. black dots, 2.5 mm. in diameter and 14.5 mm. apart. Both Foerster's and Aubert's eyes were tested, and eight meridians were explored in each eye. Irregular horizontal oblongs were obtained which differed both between the subjects and for the separate eyes, showing that the capacity of the retina to perceive distinctly two points decreases very unequally in the various meridians, and it is different for each

eye.

From the time Aubert and Foerster published their work, nothing appeared in the literature on the acuity of indirect vision till 1873. In that year Dor published a study on Beiträge zur Electrotherapie der Augenheilkunde, in which he gave determinations of the acuity from the center out as far as 40 degrees. During the following ten or twelve years a large number of studies appeared, but since 1884 little activity has been manifest in this particular field.

All the investigators during this period aimed to determine the acuity of peripheral parts of the retina relative to the center. They all used the perimeter method, with constant illumination, and they varied little from the lines laid out by Hueck, Volkmann, and Aubert and Foerster. As units of vision Dor,' Dobrowolsky and Gaine, and Hirschberg used Snellen test types; Matthiessen' used six letters from Monoyer's table; Schadow" used the letter E; Königshöfer" used black dots; and Landolt,' Butz, and Charpentier used black squares of different sizes.

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The results obtained vary no more than one would expect from the different procedures followed and from the individual differences of the subjects. Within 15 degrees of the center all the investigators found little difference between the inner and the outer radii, and between the upper and the lower. Beyond 15 degrees the fields were usually found to be largest on the lateral side, with the inner, lower, and upper sides coming in the order mentioned. Landolt found the lower field to be the largest, with the lateral next; Königshöfer found

1 H. Dor: Archiv f. Ophth., 1873, 3, p. 321f.

'Dobrowolsky and Gaine, 'Uber die Sehschärfe (Formsinn) an der Peripherie der Netzhaut,' Archiv. f. d. Gesamte Physiologie, Bd. XII, 1876.

Hirschberg, 'Uber graphische Darstellung der Netzhaut function,' Archiv. f. anat. u. Physiol, 1878.

♦ Ludwig Matthiessen, ‘Ueber die rädiäre Ausdehnung des Sehfeldes und die Allometropie des Auges bei indirectem Sehen,' Arch. f. Ophth., XXX, 1884.

'Schadow, 'Die Lichtsempfindlichkeit der peripheren Netzhautteile im Verhälniss zu deren Raum und Farbensinn,' Archiv. f. d. ges. Physiol., XIX, 1879, P. 439f.

Oskar Königshöfer, Das Distinctionsvermögen der peripheren Teile der Netzhaut, Inaug. Dissert., Erlangen, 1876. (Data obtained from Charpentier's review.)

7 ' Landolt, Handbuch der gesamte Augenheilkunde, Graefe und Laemisch, 1874, III. (Data obtained from Charpentier's review.)

* R. Butz, ‘Untersuchungen über die physiologische Function der Periph

erie der Netzhaut,' Arch. f. Anat. u. Physiol., 1881, p. 437f.

'A. Charpentier, 'De la vision avec les diverses parties de la retine,' Archiv.

de Physiol., norm. et pathol., IV, 1877, pp. 894-945.