SUMMARY We studied how the ratio (K) of the internal:external diameter of human femora follows the biomechanical optima derived earlier by other researchers for marrow-filled tubular bones with circular cross section and minimum mass designed to withstand yield and fatigue, or stiffness, or bending fracture, or impact strengths. With evaluation of radiographs of 107 femora from 57 human mummies the values of K were measured. We found that Kposterior=0.498±0.085 for the posterior radiographic view, and Kmedial=0.589±0.070 for the medial view with Kmin=0.345 and Kmax=0.783. The theoretical optima for K depend on the ratio (Q) of the marrow:bone density. Accepting the assumption of earlier authors that Q=0.50, our data show that human femora are optimised to withstand bending fracture, or yield and fatigue strengths. There were no sex-, age- and length-specific differences in K, and the means of K of the right and left femora of individuals were statistically not significantly different. The biomechanical optimization for K of human femora is not finely tuned. Compared with fox femora, K of human femora follows the biomechanical optimum to a much lesser extent. Although the relative wall thickness W=1–K of human femora are optimised, the very low relative mass increment due to deviation of K from the optimum and the considerable intraspecific variance of K make it probable that an accurate optimization of the relative wall thickness is irrelevant in humans.

The optimum for the ratio K of the internal to external diameter of a marrow-filled tubular bone with minimum mass designed to withstand a given type of strength (yield/fatigue, stiffness, fracture or impact) depends on Q=ρm/ρb only, where ρm and ρb are the densities of marrow and bone. With computer-assisted evaluation of radiographs of 62 femurs in the red fox (Vulpes vulpes) we measured the values of K. The mean and standard deviation of K are 0.68 and 0.036, and K changes in the rather wide range from 0.59 to 0.74. Accepting the assumption of earlier authors that Q=0.50 or 0.44, our data would support the hypothesis that the fox femurs are optimized to withstand yield, fatigue or stiffness strengths. However, since the Q-values are unknown, the possibility cannot be excluded that any studied fox bone with an appropriately selected Q-value is optimized for any strength type. Assuming Q=0.50 or 0.44, the relative mass increments μ of the investigated fox bones are smaller than 5% under all four mechanical conditions. The evolutionary relevance of such tiny μ-values is questionable.

SUMMARY Using 180° field-of-view imaging polarimetry, we measured the reflection-polarization patterns of two artificial surfaces (water-dummies) in the red, green and blue spectral ranges under clear and partly cloudy skies at different solar elevations. The dummies consisted of a horizontal glass pane with a matt black or matt light grey cloth underneath, imitating a dark or bright water body, respectively. Assuming that polarotactic water insects interpret a surface as representing water if the degree of linear polarization of reflected light is higher than a threshold and the deviation of the direction of polarization from the horizontal is lower than a threshold, we calculated the proportion, P, of the artificial surfaces detected polarotactically as water. We found that at sunrise and sunset P is maximal for both water-dummies and their reflection-polarizational characteristics are most similar. From this, we conclude that polarotactic water detection is easiest at low solar elevations, because the risk that a polarotactic insect will be unable to recognize the surface of a dark or bright water body is minimal. This partly explains why many aquatic insect species usually fly en masse at dusk. The daily change in the reflection-polarization pattern of water surfaces is an important visual ecological factor that may contribute to the preference of the twilight period for habitat searching by polarotactic water insects. Air temperature at sunrise is generally low, so dusk is the optimal period for polarotactic aquatic insects to seek new habitats.

Using imaging polarimetry, we measured the polarization patterns of a rainbow on the shore of the Finnish town of Oulu in July 2001. We present here high-resolution color-coded maps of the spatial distributions of the degree and angle of linear polarization of the rainbow in the red (650 +/- 30 nm), green (550 +/- 30 nm), and blue (450 +/- 30 nm) ranges of the spectrum. The measured polarization characteristics of the investigated rainbow support earlier theoretical and computational results and are in accordance with previous qualitative observations. To our knowledge, this is the first imaging polarimetric study of rainbow polarization.

In the clear sky there are three commonly known loci, the Arago, Babinet, and Brewster neutral points, where the skylight is unpolarized. These peculiar celestial points, bearing the names of their discoverers, have been the subject of many ground-based investigations, because their positions are sensitive indicators of the amount and type of atmospheric turbidity. According to theoretical considerations and computer simulations, there should exist an additional neutral point approximately opposite to the Babinet point, which can be observed only at higher altitudes in the air or space. Until now, this anonymous "fourth" neutral point has not been observed during air- or space-borne polarimetric experiments and has been forgotten, in spite of the fact that the neutral points were a basic tool in atmospheric research for a century. Here, we report on the first observation of this fourth neutral point from a hot air balloon. Using 180 degrees-field-of-view imaging polarimetry, we could observe the fourth neutral point at 450, 550, and 650 nm from different altitudes between 900 and 3500 m during and after sunrise at approximately 22 degrees - 40 degrees below the anti-solar point along the anti-solar meridian, depending on the wavelength and solar elevation. We show that the fourth neutral point exists at the expected location and has characteristics similar to those of the Arago, Babinet, and Brewster points. We discuss why the fourth neutral point has not been observed in previous air- or space-borne polarimetric experiments.

SUMMARY 1. Based on the findings that some dragonflies prefer either ‘dark’ or ‘bright’ water (as perceived by the human eye viewing downwards perpendicularly to the water surface), while others choose both types of water bodies in which to lay their eggs, the question arises: How can dragonflies distinguish a bright from a dark pond from far away, before they get sufficiently close to see it is bright or dark?2. Our hypothesis is that certain dragonfly species may select their preferred breeding sites from a distance on the basis of the polarisation of reflected light. Is it that waters viewed from a distance can be classified on the basis of the polarisation of reflected light?3. Therefore we measured, at an angle of view of 20° from the horizontal, the reflection‐polarisation characteristics of several ponds differing in brightness and in their dragonfly fauna.4. We show that from a distance, at which the angle of view is 20° from the horizontal, dark water bodies cannot be distinguished from bright ones on the basis of the intensity or the angle of polarisation of reflected light. At a similar angle of view, however, dark waters reflect light with a significantly higher degree of linear polarisation than bright waters in any range of the spectrum and in any direction of view with respect to the sun.5. Thus, the degree of polarisation of reflected light may be a visual cue for the polarisation‐sensitive dragonflies to distinguish dark and bright water bodies from far away. Future experimental studies should prove if dragonflies do indeed use this cue for habitat selection.

Because the human eye is practically blind to the polarization of light, biologists dealing with polarization vision of animals, or engineers designing robots using polarization-sensitive computer vision to enhance contrast in the optical environment need a technique to image the spatial distribution of polarized light in the visual environment. Recently, different kinds of imaging polarimetry were developed to measure the polarization patterns of objects and natural scenes in a single, two-dimensional, wide field of view. As a further development of this technique, we report here on the realization of the addition of depth to scenes imaging the distribution of polarized light: One kind of stereo videopolarimetry was designed to measure and visualize in three dimensions the polarization patterns in nature and to mimic the ability of animal-eyes to receive visual information from a binocular field of view. We demonstrate the power of stereo videopolarimetry on an applied problem representing (in parallel view stereo format) a three-dimensional object, a car with a shiny bodywork and also having strong reflection polarization. The technical difficulties and hitches of stereo videopolarimetry as well as the importance of the distance of observation, the role of the angle of view, the influence of the color of the object, and the possibility to state differences between metallized and non-metallized paints are discussed.

A computer assisted method of exact automatic measurement of BAEPs, mainly for the purpose of localization of brain-stem lesions, is described. The method presents analogue curves with superimposed confidence regions for the amplitudes and peak latencies of separate waves. The numerical output presents peak latencies, amplitudes, interpeak time intervals and central conduction times, all with normative values (60 healthy subjects), and ±2 and ±3 S.D. intervals and automatic labelling of pathological parameters (outside the chosen statistical limits). With the exception of the identification of separate waves in the often greatly distorted pathological BAEP the whole processing and display of the results are fully automatic.On décrit une méthode assistée par ordinateur, permettant une évaluation exacte et automatique des potentiels évoqués auditifs du tronc cérébral, avec pour but plus spécifique la localisation de lésions du tronc cérébral. Le dispositif présente des courbes analogiques des amplitudes et des latences des différentes composantes, avec limites de confiance surajoutées. La sortie numérique fournit les valeurs normatives (pour 60 sujets sains), des latences des pics, d'amplitude, d'intervalles interpics et de temps de conduction centrale (avec ±2 et ±3 ES). Elle indique automatiquement les paramètres pathologiques — qui se situent en dehors des limites statistiques choisies. A l'exception de l'identification d'ondes séparées, en cas de potentiels pathologiques très déformés, l'ensemble des évaluations et la présentation des résultats sont entièrement automatiques.