A review of a family and the most recent case history in the only male has shown that the cerebellar tumdur was verified for sibling I at the age of 29 years by pathological section and an eye tumour was treated by radon seeds. Sibling II-the cerebellar tumour was verified at operation at the age of 23 years and by pathological section at two subsequent operations. The eye tumour presented a similar pathological appearance. Sibling III had a naevus of the limbus of the left eye, which became less apparent after nine years. Sibling IV: The new case was verified by pathological examination at the age of 28 years and after extensive radiation an-eye tumour, capillary in type, has slightly progressed in four years. Acknowledgment: To Dr. K. W. McKenzie my thanks and congratulations for the surgical care and remarkable results that have meant so much to the members of this family. Dr. Eric Linell was responsible for the excellent pathological reports and photographs. Miss Wishart made coloured drawings, which add much to the written description. Also thanks to the Departments of Surgery, Neuro-Pathology, Veterans' Affairs, Ophthalmology, Art and Photography.

In 1894 K6nig noticed that if the size of a coloured object was sufficiently reduced and that if it was observed by the eye so that its image fell upon the central fovea, all colours could be matched by a mixture of a red and a blue stimulus only, i.e. that vision under these conditions was dichromatic. K6nig also discovered that the ability to differentiate between wavelengths in the blue-green region of the spectrum was impaired and that the type of vision was similar to that found in the tritanopic form of colour blindness.Recently, Willmer (1944), and Willmer & Wright (1945) have confirmed and extended K6nig's observations. Luminosity, dichromatic coefficient and huediscrimination curves were measured and spectral mixture curves calculated for a small circular field whose diameter subtended an angle of 20' at the eye. The image of this field lay on the retina at the fixation point within the central fovea, and vision for this field was found to be dichromatic and tritanopic. Fixation of the object could only be maintained momentarily and if fixation was imperfect the dichromatic match was at once disturbed. Hartridge (1945a, b), using as his test for dichromatic vision the fact that yellow is seen as white, has also investigated the sensitivity of the fovea with a series of small yellow-painted test objects, and compared this sensitivity with that of the surrounding retina. He concluded that if an object were sufficiently small, it would exhibit the characteristics of dichromatic vision not only at the fovea but wherever its image lay on the retina. It is clear that in his view, retinal position is of no importance, and that it is the reduction of size alone which initiates the effects described by Konig. He inferred from his experiments that there was no significant difference between vision at the foveal centre and that within a much larger retinal area.The present series of experiments were designed to record the characteristics of vision for different points within the central fovea. METHODThe observing fields were of two types. In type A, Fig. 1, the test and comparison beams formed the two halves of a small circular field, the diameter of which subtended an angle of 15' at the eye. Displacement of the field was effected by the provision of two points of light to one side of the circular field so that, if the appropriate point was fixated, the centre of the circular field would then be displaced from the fixation point through an angle of either 20' or 40'. In type B, Fig. 1 (Wright, 1927(Wright, , 1939 in which a wide variety of spectral lights, with independent intensity control, may be applied to each of the two halves of the circular field. Correct alinement and fixing of the head relative to the exit pupil of the instrument were obtained by biting upon a suitably placed dental impression mouthpiece. A further addition to the optical system was the introduction, immediately behind the exit pupil of the colorimeter, of a small lens with sufficient chromatic over-correction to correct for the chromat...

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The question 'Does simultaneous stimulation of the retina of both eyes give rise to a greater response than stimulation of one eye only?' has interested investigators since the last century. Lythgoe & Phillips (1937), who give references to previous authors, measured the change in light threshold during dark adaptation for right, left and both eyes and found that the binocular threshold was lower than the monocular. In both foveal and peripheral vision the binocular threshold was approximately the same as the monocular threshold for a test patch of twice the area, a finding which showed that a moderate degree of summation had occurred when both eyes had been used. Crawford (1940) has repeated the work of Lythgoe & Phillips with substantially similar results and, in addition, has measured the minimum detectable difference of brightness at various brightness levels for both eyes and for a single eye. These measurements showed only a small amount of summation at all brightness levels when the central fovea was used. With the parafovea (80) two eyes were about 30 % more sensitive than one eye to differences of brightness, except at low brightness levels when only a small amount of summation was obtained. Clearly the brightness threshold behaves in a different manner from the light threshold. Crawford gives no explanation of this difference. Pirenne (1943), using a new method of light threshold measurement, has found that there is a small degree of summation in the peripheral retina (200). He presented several series of 4 m./sec. flashes of light to the eye and for each series determined the number of flashes seen by the observer. The 'frequency of seeing' with both eyes was somewhat greater than that obtained with one eye only, but doubling the intensity of the flashes and observing with a single eye produced a much larger increase in the 'frequency of seeing', and he concluded that under the conditions of his experiment binocular summation was small in amount. Thus it seems that the degree of summation of afferent impulses derived from two eyes depends upon the way in which the effect of these afferent impulses is to be measured. The object of the present experiments was to find out if summation of afferent impulses occurs when a purely objective effect, such as contraction of the pupil, is measured. Since the nervous pathways of the pupillary light reflex do not ascend higher than the mid-brain, the results might help to localize the site of summation within the nervous system.