M. J. Koomen

The Spherical Aberration of the Eye

The spherical aberration of the eye was measured by placing a series of centered annular apertures over the eye pupil, and determining the optimum spectacle correction for each aperture. A “double star” was used as a test object. Accommodation was controlled by reflecting a second test object into the field of view. The three eyes examined had positive (undercorrected) spherical aberration when unaccommodated; in one case 2 diopters at the pupil margin. The aberration was reduced with increasing accommodation and in one case became negative at high accommodation. Homatropine reduced the spherical aberration of two of the three eyes examined. A historical review of previous work is given.

A Study of Night Myopia

The phenomenon of night myopia, wherein the eye becomes relatively nearsighted in dim light, was investigated in detail using high contrast grating test objects. Night myopia first appeared at the brightness level where rod vision began to take place and grew larger as the brightness was further reduced. At the lowest brightness investigated, the myopia attained a value of 1.5 to 2.0 diopters, depending upon the observer. Night myopia appeared when accommodation was prevented by an optical method and also when accommodation was paralyzed with homatropine. It was therefore concluded that accommodation was not a significant cause of night myopia in the observers examined.The spherical aberration of the observers’ eyes was measured, and its effect upon the effective focal length of the eye was investigated with the aid of artificial pupils. Also studied were the properties of a simple glass lens having spherical aberration approximating that of the eye. All tests showed that night myopia, and its dependence upon the brightness level, is primarily a result of undercorrected spherical aberration of the eye. For some eyes, homatropine reduced night myopia slightly, but only to the extent that it reduced the spherical aberration. A review of the literature is included.

White Light Coronagraph in OSO-7.

A small, externally occulted Lyot-type coronagraph, designed for use in the seventh unmanned Orbiting Solar Observatory (OSO-7), is described. Optical configuration, suppression of stray light, SEC vidicon detector, and data system are discussed, as well as integration of the instrument into the spacecraft and operation in orbit. Orbital operation produced daily images of the white light corona, from 2.8 to 10 solar radii, at least once per day for 2(3/4) yr. The first records of white light coronal transient events were obtained, and the corona was shown to be constantly changing.

Measurements of the Brightness of the Twilight Sky

With a recording photometer of photopic sensitivity, measurements were made of many points in the sky during twilight for solar altitudes H=+5° to −15° for clear air and no clouds at two stations, one in Maryland, altitude 30 meters, and one on Sacramento Peak, New Mexico, altitude 2800 meters. The sky polarization on the meridian through the sun, and the illumination on a plane at various orientations exposed to the sky, were also recorded. For H from about −3° to −11° the entire sky changed in brightness at about the same rate of a factor of 10 for each 2° change in H. Except at the horizon the Sacramento Peak sky was about 23 to 12 as bright as the Maryland sky because of clearer air; at the horizon the two were about the same. At Sacramento Peak the ratio of the polarized components reached a minimum of about 0.06 at the zenith for H=−3°.

Measurement of Accommodation in Dim Light and in Darkness by Means of the Purkinje Images

The state of focus of the eye in dim light and in total darkness was investigated by photographing the Purkinje images of a double aperture placed over a high speed flash lamp. Measurement of the photographed images showed that all subjects, when viewing a dim scene, remained focused for far vision. By subjective test, however, these subjects were myopic under the same conditions of dim light. The experimental results thus favor the theory that “night myopia” is a result of the aberrations of the eye and is not due to accommodation. The flash photographs taken in total darkness showed that three subjects remained focused for far vision; the fourth subject sometimes accommodated, by amounts varying between 0.5 and 1.5 diopter.

The Visibility of Stars and Planets During Twilight

From known values of twilight sky brightness, atmospheric transmission, and eye sensitivity, the visibility of planets and stars of magnitudes −2.5 to +4.5 is calculated for the twilight period. The results of the calculations are given in charts which can be applied to various altitudes of the observer and various conditions of observation. The twilight belt is about 1100 sea miles in width, from the sunset or sunrise meridian to full night. The charts show that under good conditions in the first 200 miles of the belt the brighter planets and Sirius become visible, in the next 200 miles first magnitude stars appear, and beyond 500 miles practically all navigational stars can be seen.

An Infra-Red Pupillometer

A pupillometer for measuring pupil diameters in total visual darkness has been constructed. It consists of an infra-red electron telescope arranged to observe the eye from a distance of about six inches. An attached light source irradiates the eye with invisible infra-red radiation. A scale is projected into the field of the instrument to measure the pupil diameter directly. The instrument can be used in dim and bright visible light as well as in darkness. Other possible uses of the instrument for infra-red examination of the eye are suggested.

Visibility of Stars at High Altitude in Daylight

The daylight visibility of stars has been investigated for an observer altitude of 100 000 ft, using published visual threshold data and calculated sky luminance. Venus, Jupiter, and Sirius, plus Mars at its brighter phases, can be detected with the naked eye if the observer knows where to look for them in the sky. Saturn and Canopus may be seen only under rare circumstances. A random search of the sky will reveal neither planets nor stars except possibly Venus. If a 10-power telescope of large exit pupil is used, there is a possibility of detecting an occasional star by careful search of the sky. The daytime sky will not exhibit nighttime luminance until an altitude of roughly 100 km has been reached, assuming no contribution from airglow.