The stability of corneal topography in the post-blink interval.

PURPOSE: Videokeratoscopes provide a wealth of information about the topography of the ocular surface. Although there have been numerous studies of the accuracy and precision of videokeratoscopes with inanimate test objects, little information exists on their precision (repeatability) for real eyes. METHODS: To investigate the stability of the ocular surface in the inter-blink period, 10 patients were recruited for videokeratoscopy. Tear break-up time was measured and videokeratographs were acquired immediately post-blink and again at 4, 8, and 12 seconds post-blink. To permit statistical inferences to be drawn from the data, we acquired 24 videokeratographs for each of the four post-blink intervals. The videokeratograph data were interpolated (bilinear) to a common grid, and average and standard deviation (SD) maps were derived for each post-blink condition. t Tests were used to test the significance of changes observed in the topography. RESULTS: The instantaneous power SD maps showed increasing variation toward the periphery, with most maps showing less than +/-0.5 diopters (D) of SD in the central 4 to 5 mm and variation in the periphery often reaching more than +/-1 D SD at the edge of an 8-mm diameter. When the 4-, 8-, and 12-second average maps were subtracted from the average map acquired immediately after blinking, regions of statistically significant ( p < 0.001) change were apparent in the upper and lower regions of the maps. The upper and lower bands of change were found to correlate with the natural position of the patients' lid margins. CONCLUSIONS: For normal eyes, the central regions of videokeratographs show high stability in the inter-blink period. However, the upper and lower edges of 8-mm diameter maps show statistically significant variability, which appears to be related to the effects of eyelid pressure.

The visby lenses.

PURPOSE: In this study, 10 lens-shaped rock crystals, manufactured not later than the early Middle Ages, have been examined with respect to their image-forming qualities. METHODS: The spherical aberration of the lenses served as a scale for comparison. Measurements have been taken with a specially designed light section method. Some of the examined lenses have a silver mounting and have been used as pendants, whereas others are unmounted and show no signs of use as jewelry. RESULTS: The two largest unmounted lenses have very interesting surface curvatures, rendering a very small spherical aberration of the lens. The combination of top and bottom surfaces of the lens causes an imaging quality comparable to the modern aspheric lenses used, e.g., in today's projectors. CONCLUSION: According to the results of this study, the knowledge and comprehension of optical design was much further developed in the Middle Ages than we assume today.

The scintillating grid illusion.

Disk-shaped luminance increments were added to the intersections of a Hermann grid consisting of medium grey bars on a black background. Illusory spots, darker than the background, were perceived as flashing within the white disks with each flick of the eye. This striking phenomenon may be referred to as the scintillating grid illusion. We determined the conditions necessary for cancelling the Hermann grid illusion, as well as the luminance requirements and the size ratio between disks and bars that elicits the scintillation effect. The fact that scanning eye movements are necessary to produce the scintillation effect sets it apart from the Hermann grid illusion.

Temporal properties of brightness and color induction.

With a matching procedure, we studied the temporal properties of direct brightness (or lightness) and chromatic changes (produced by modulation of the region being matched) and induced brightness and chromatic changes (produced by modulation of the surround of the region being matched). The amount of direct brightness and color change was found to vary only slightly with temporal frequency over the 0.5-8 Hz range studied, whereas induced changes were found to occur only at low temporal frequencies, below about 2.5 Hz. With high temporal-frequency modulation of the surround, the induced patterns appeared to flicker but not to change in brightness or color. Despite the fact that chrominance and luminance temporal contrast sensitivity functions are very different, the temporal induction curves for color and brightness were very similar. However, brightness induction was found to increase approximately linearly with increasing surround modulation up to very high levels, whereas the amount of color induction was much less dependent on the modulation depth of the surround.

[Studies on the precorneal tear film. First communication: an optical system for demonstrating tear film without fluorescein (author's transl)]. / Studien über den präkornealen Tränenfilm (PKTF). 1. Mitteilung. Ein optisches Verfahren zur Darstellung des Tränenfilms ohne Fluoreszein.

Tear film is usually studied by using fluorescein. An optical system is described which despenses with the use of fluorescein. It mainly consists in an equal diffuse illumination of the cornea, provided through an opaque truncated cone that is illuminated from behind. The cone is situated as close as possible to the front of the eye. The cornea and its surface can be observed through a hole at the narrow end of the cone. Interferences in the lipid layer of the tear film are recognizable as "colors of thin layers". The physical background to interferences in thin, convex layers is discussed. Break-ups are visualized by using a supplementary "Schlieren" method.

[Critical review of our knowledge and presentation of a new working hypothesis concerning the stability of the precorneal tear film (PCTF) (author's transl)]. / Kritische Ubersicht über unsere Kenntnisse und Vorstellung einer neuen Arbeitshypothese über die Stabilität des präkornealen Tränenfilms (PKTF).

1. The paper reviews critically our present knowledge and experimental findings concerning the structure, formation and stability of the PCTF. 2. A new hypothesis, which contradicts Holly's concept, is put forward to explain the rupture of the PCTF (formation of dry spots). 3. Special chapters deal with tear secretion and drainage, and finally with the function of the eyelids (blinking), especially in relation to the formation and preservation of a stable preocular tear film.

Psychophysical measurements concerning the range of visual perception.

Two adjacent semicircular testfields of luminances l and L + delta L, were presented on an adapting field of luminance LU. In earlier experiments, it was found that the just noticeable difference of brightness (jnd) delta L, can be described by delta L = delta Ls + bLc (Lingelbach and haberich 1977). For foveal observations and LU values within the photopic range, delta Ls and b are power functions of LU. In this study, these investigations area continued for scotopic background luminances and extrafoveal conditions (6 degrees and 12 degrees temporal). The following results area obtained: 1. Under scotopic conditions of background luminance, LU, the jnd's for foveal observations are nearly independent of LU. delta L is dependent on the luminance L and can be expressed by the same curve for different levels of LU. 2. For extrafoveal conditions, the results can be described in the same way as for the fovea. The jnd is found to be delta L = delta Ls + bLc. In the total range of LU investigated (0.001 less than or equal to LU less than or equal to 100 cd/m2), the parameters delta Ls and b are power functions of LU. c increases with log L from about 1.1--1.5. 3. The sum of jnd-steps as separate "units of sensation" form a "dynamic characteristic" for each adaptational level. It is different for fovea and periphery. 4. The Fechner scaling of brightness, which corresponds to the sum of jnd-steps up to each adaptational luminance LU ("static characteristic"), shows an unexpected similarity to the Stevens power function.

Intestinal absorption under the influence of vasopressin: studies in unanaesthetised rats.

Experiments were carried out in different segments of the intestine of unanaesthetised rats to assess the effect of vasopressin on intestinal absorptive processes. The following data were observed. (1) Within a physiological range of doses (Aziz, 1969), ADH diminished the net sodium absorption mainly by reducing the unidirectional sodium influx, whereas the behaviour of the efflux was not uniform. (2) The unidirectional volume fluxes showed the same behaviour as did the sodium fluxes. (3) ADH produced an oral-aboral gradient (jejunum greater than ileum greater than colon). (4) ADH did not significantly change the transfer of actively transported sugars; it did influence, however, passively transported substances. (5) During the intravenous application of ADH, a substance was secreted into the perfusion solution which diminished the absorption of volume and electrolytes. (6) Cyclic AMP acted on intestinal absorption in the same way as did ADH. In view of these results two mutually independent transport pathways for sodium and water are supposed, one of which is influenced by ADH or cAMP. Based on a two membrane model, an ADH mechanism is discussed: the permeability of the luminal membrane system is enhanced in the presence of vasopressin.