BLUE EYES w. MASON’ For a long time it has seemed highly probable that blue eyes owe their color to structure rather than to a blue pigment. Brucke2 appears to have made the first statement regarding this. “There is no blue pigment in the iris of blue-eyed persons; but it looks blue because there is a dark pigment behind the whitish, transparent tissue. Later he3 recognized the blues of turbid media in an extensive paper; but did not apply this to the question of blue eyes. In 1866 Helmholtz4 stated that “the stroma may contain pigment, resulting in brown; otherwise it appears blue like a turbid medium in front of a black background.” Tyndal15 and Rood6 have concurred in this. Quite recently Bancroft’ has assembled the statements of some other writers on this subject and has pointed out the conflict of their views with the older ones, and the necessity for an experimental study of the subject. Roberts8 says: “The iris, on which the color of the eye depends, is a thin membranous structure composed of unstriped muscular fibers, nerves, and blood-vessels, held together by a delicate network of fibrous tissue. On the inner surface of this membrane there is a layer of dark purple pigment called the uvea (from its resemblance to the color of a ripe grape), and in brown eyes there is an additional layer of yellow( and perhaps brown-red) pigment on its outer surface also, and in some instances there is a deposit of pigment among the fibrous structures. In the albino, where the pigment is entirely absent from both surfaces of the iris, the bright red blood is seen through the semitransparent fibrous tissues of a pink color, and in blue eyes, where the outer layer of pigment is wanting, the various shades are due to the dark inner layer of pigment-the uvea-showing through fibrous structures of different densities or degrees of opacity. The eyes of new-born infants of both white and black races (and I believe the new-borp of all the lower animals) are dark blue in consequence of the greater delicacy and transparency of the fibrous portion of the iris; and as these tissues become thickened by use, and by advancing age, the lighter shades of blue, and finally gray are produced; the gray, indeed, being chiefly due to the color of the fibrous tissues themselves. I n gray eyes, moreover, we see the first appearance of the superficial layer of yellow pigment RY CLYDE
The investigation upon which this article is based was supported by a grant from the Heckscher Foundation for the Advancement of Research, established by August Heckscher at Cornel1University. * Site. Akad. Wiss. Wien, Abt.1, 7, 802 (1851). 3 Brucke: Sita. Akad. Wiss. Wien, Abt.1, 9,530 (1852). 4 Handbuch der physiologischen Optik, 1, 15 (1866). ‘Phil. Mag. (4) 37, 393 (1869);“Fragments of Science: The Sky” (1888). 6 (‘Modern Chromatics”, 58 (1879). J. Phys. Chem. 23,356 (1919). Brit. Ass. Reports. 50, 135 (1880).
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in the form of isolated patches situated around the margin of the pupil, or in rays running across the iris. I n the hazel and brown eyes the uvea and the fibrous tissues are hidden by increasing deposits of yellow and brown pigment on the anterior surface of the iris, and black eyes result when this is very dense. It is very doubtful, however, whether the iris is ever so dark colored in the inhabitants of this country as to justify the term black being applied to it, and the popular use of the expression has reference to the widely dilated pupil common in persons with dark brown eyes. The nearest approach to a black eye among us is the dark blue or violet eye associated with black hair in some Irish adults; here the color is probably not due entirely, as in infants, to the greater transparency of the fibrous structures, but to interstitial deposits of black pigment, or to a layer situated on the anterior surface of the iris.” This view-point seems to have been adopted by the biologists1 and to be disputed by the medical men. One physician writes that “the color of the iris is dependent upon two factors, namely, the amount of pigment in the pigment epithclium on the back of the iris and the amount and distribution of the pigment in the stroma of the iris. The pigment granules are all practically of the same color, dark brown. [The layer-of dark purple pigment, called the uvea from its resemblance to the color of a ripe grape, is apparently a myth.] I n some albinos the iris is entirely free from pigment and appears pink. I n all other cases the pigment epithelium is so densely packed with pigment that, seen by itself, it appears black. A blue iris appears so because the stroma is free from pigment and the pigment epithelium is seen through the translucent stroma and appears blue just as veins appear blue when seen through a delicate skin. All other colors of the iris are dependent upon the amount of pigment in the stroma; if this is abundant, as in the negro, the iris appears black. Practically all infants are born without any pigment in the stroma so that their eyes are blue. The thickness of the iris varies in different eyes and in different places in the iris, and also varies with the dilatation of the pupil. After Zenker’s fixation there is practically no shrinkage of the iris, and measurements that I have made after such fixation show the iris to measure 0.4-0.6mm in thickncss. I know of no measurements made on frozen sections of fresh eyes.” Pouchet considered the‘blues of turbid media exhibited by various tissues to be related to fluorescence and named the phenomenon cdrulescence. MandouP takes issue with him and has established the nature of Tyndall blues in a considerable variety of animal tissues. This diversity of opinion, together with the apparent absence of any actual experimental data for either side, seems to justify further study. The determination of the nature of the blue colors in feathers3 involved the development of methods which are applicable to the identification of structural blues in other subjects, and the same criteria may be employed. Darbishire : “Breeding and htendelian Discovery,” 41 (191I ) ; Davenport: “Heredity in Relation to Eugenics, 27 (1911). *Ann. Sci. Nat. Zool. (8) 18, (1923). Mason: J. Phys. Chem. 27,215 (1923).
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CLYDE W. MASON
The most characteristic and most readily observed properties of the socalled “Tyndall blues” (of turbid media) are : I. Minute particles, < o . 6 ~ of , different refractive index from the surrounding medium. 2. Scattered light is blue; transmitted light is yellowish. 3. Scattered light more or less polarized; vibration in plane normal to the direction of the incident beam. Other properties, such as change on swelling, permeation, pressure, etc. are applicable only when the particles are pores or cavities. The aim of this investigation was to demonstrate either that a blue pigment was present, or that the blue was of the typical Tyndall type, analogous to the blue of the sky, smoke, skimmed milk, blue feathers, etc. Human material was not available but the same methods may be applied to it as were employed in this study. Mr. Irvine H. Page kindly furnished us with specimens and aided in the dissection. Several eyes of two weeks old kittens were studied. These were of a clear blue, rather better than the blue eyes of most people. (It should be mentioned that very few blue eyes are really better than a blue gray. The blues do not compare in brilliance or purity with those of the other blue media mentioned above). The material was examined within four hours of death, was not treated with any preservative, and physiological saline solution was used as a mounting medium. Observations with transmitted light were carried out with daylight, while the scattered light was observed with a dark-field illuminator, and also by sending a horizontal beam of light through the preparation in the field of the microscope, a concentrated-filament lamp and hulls-eye condenser being employed. Polarization was observed by placing a cap nicol prism over the ocular of the microscope, and revolving this to the position of minimum intensity of the light scattered by the preparation. A selenite plate (“1st. order red”) gives a red to green color change when placed below the nicol in the above system. The horizontal beam across the field of the microscope was employed for this observation. Care was taken to distinguish between effects due to the irregularities of the external surface and those due to the polarizing action within the tissue itself. The iris was separated into the stroma layer, and t’he black pigment layer (uvea). The stroma, by transmitted light was turbid yellow-of the same hue as that of other turbid media; the color was not localized in pigment granules. Against a dark ground, the stroma scatters whitish blue light and with a 3 mm objective a haze of tiny points of blue light was observed. The scattered light was partially polarized, and the vibrations were in the plane normal to the direction of the illuminating beam. No evidence of a blue pigment was noted; the yellow appearance by transmitted light would preclude the possibility of such a pigment being present. The sclerotic layer (“white” of the eye) was of a turbid, bluish white, and microscopic study revealed the same optical characters as in the case of the
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iris. The absence of marked surface irregularities renders thr examination simpler than that of the iris. The pigment of the uvea is not purple but dark brownish black (melanin) and serves as a background for the turbid stroma. In like manner the choroid serves as a dark background for the sclera though the latter is frequently so thick and so opaque that the dark background is really not necessary and the sclera appears almost white. The retina presents a bluish gray appearance, and shows the properties of a Tyndall blue medium, though to a less degree than ihe stroma or sclera. This is no doubt due to the scattering of light by the various minute structures of the retinal nerve endings. The “visual purple” is another thing. It is a true pigment, has been isolated and studied chemically, and it fades rapidly in the light’. Blue pigments appear to be very rare, except in the vegetable world, and it is probable that most blue and bluish-white turbid substances (tissues, etc.) are simply examples fo the Tyndall blue. Their nature might be established by simple tests as outlined above. It would also be interesting if some one with an abundance of experimental material would compare different shades of blue eyes and blue eyes of persons of different ages, to establish a more definite relationship between the size and number of the particles of the turbid layers and the color of the eye. A study of the increase in whiteness and opacity of the sclera with age might also be interesting. This structural basis of the blue of eyes fits the observed fact that the blues generally become lighter and grayer with age, for a slight increase in the size of the tiny particles would accomplish this. The development of a yellow to brown pigment in the turbid stroma would of course give shades ranging from green to hazel or brown. The conclusions of this paper confirm those of Bancroft’s article2 and are as follows: I. In blue eyes there is no pigment in front of the uvea, which is brownish black (melanin). 2. The blue color is the color of turbid media (Tyndall blue) and is localized in the stroma. 3. The w e n serves as a dark background and permits the maximum appearance of blue from the turbid stroma. 4. Pigmentation in the stroma may combine with the blue to give green, hazel, or brown eyes. 5 . Increase in the size of the particles of the turbid layer would account for lessened clearness of the blue with age. 6. The sclera consists of a thick layer of dense whitish Tyndall blue, backed by the black choroid. Cornell University. August 1, 1922. Ayers: N. Y. Med. Jour. 1881, 582. J. Phys. Chem. 23,356 (1919).
