Influences of natural dissolved organic matter on the interaction of aluminum with the Microalga Chlorella: a test of the free-ion model of trace metal toxicity

The free-ion model of trace metal interactions with aquatic microorganisms states that the biological response to a metal is proportional to the activity of the free-ion {Mz+} in solution. The applicability of the free-ion model, as it applied to the toxicity of aluminum to the green alga Chlorella pyrenoidosa in the presence of a soil fulvic acid (SFA), was tested in defined media (pH 5) designed to limit Al interactions to algae and SFA. Toxicity was not proportional to the activity of Al3+, an apparent failing of the free-ion model. Fulvic acid adsorbed to cell surfaces (17 mg m-2, pH 5) and increased membrane permeability (as measured with [14C]sorbitol) whereas Al decreased membrane permeability. In addition, SFA may act as a source of phosphorus to P-deficient algae. These results emphasize the importance of considering not only the metal-complexing properties of natural dissolved organic matter but also its direct metabolic and physiological influences on algae

Modeling the Virtual Campus

The challenge facing TeleLearning and Distance Education is to support the essentially autonomous knowledge building process by the learner, while at the same time, emphasizing the very important collaborative dimension of learning. This challenge cannot be met without information technology and without the integration of all the useful synchronous and asynchronous technologies in a coherent architecture. We call such an architecture the Virtual Campus. A review of five LICEF's projects related to this architecture will be presented. Then, applying an object oriented modeling technique, we outline the modeling process, defining the actors of the Virtual Campus, their roles, the processes they are involved in and the computer tools they need to perform their roles. Upgrading tools and methods to support the Virtual Campus will open up and extend the traditional classroom in a very "real" way. It will hopefully contribute to learners persistence in distance education, thus extending the social and economic usefulness of TeleLearning

An Intelligent Support System for Course Design

The development of AGD, an intelligent support system for course design, is based on conceptual, procedural and strategic knowledge from the field of Instructional Design. Conceptual and procedural ID knowledge help define the user interface and tools in the workbench. Strategic ID knowledge is used to define the intelligent advisor componnent helping the designer in his or her tasks. This article gives an overview of the support system from the user point of view, showing how knowledge modelling serves as a basis for course design. Then we highlight the methods used to formalize the pedagogical knowledge

Épitalk, un outil générique pour la construction de systèmes conseillers

Nous présentons l'architecture d'un outil générateur de systèmes conseillers. Le système ÉpiTalk sert à développer un conseiller qui se greffe à un environnement d'apprentissage ou un système d'aide à la tâche existant sans en perturber le fonctionnement. Après un inventaire des formes que prend le conseil dans les systèmes d'apprentissage à base de connaissances et les tutoriels intelligents, nous présentons des exemples de systèmes conseillers et une première architecture qui a fait l'objet de travaux antérieurs. Puis nous présentons l'architecture et l'implantation d'un nouveau système générateur de systèmes conseillers multi-agents appelé ÉpiTalk. Ensuite, nous présentons trois applications d'ÉpiTalk qui ont été implantées ou sont en voie d'être complétées: AGD, un atelier de génie didactique, COPERNIC-2 un environnement d'apprentissage sur la démarche scientifique, et HyperGUIDE, un environnement pour la formation à distance. En conclusion, nous discutons les résultats sur quatre plans: la faisabilité d'un générateur de systèmes conseillers, la versatilité du générateur quant aux formes de conseil, son applicabilité à des conseils sur des activités de groupe et la qualité des interfaces visant à minimiser l'usage de la programmation

Focusing properties of an axicon pair

[en] The focusing properties of a so-called reflaxicon (a combination of a diverging and a converging axicon) are studied both theoretically and experimentally. Calculations of intensity distributions produced by this system are made by evaluating the Kirchhoff-Fresnel diffraction integral, first. by means of an approximate technique, the stationary phase method, then by a more exact numerical method. The calculations are presented for various planes along the axis of the axicons. The effects of the presence of the supporting mount of the axicons and of some important misalignments of the system on the distributions is also investigated. Experimental results of actual intensity distributions produced by focusing a near-fundamental Gaussian beam by such a system are also presented and are seen to be in fair agreement with numerical calculations. Such calculations would be valuable in many applications for predicting important characteristics (e.g., peak intensity, length of the focal line, degree of asymmetry) of the intensity distributions formed by optical systems containing an axicon pair as the focusing component._______ [fr] Les propriétés de focalisation d'un système optique formé d'un axicon convergent suivi d'un axicon divergent font l'objet d'une étude théorique et expérimentale. Les distributions d'intensité théoriques produites par ce système sont déterminées en résolvant l'intégrale de diffraction de Kirchhoff–Fresnel, d'abord de façon approximative, par la méthode de la phase stationnaire, puis, d'une manière plus exacte, par le recours au calcul numérique. Ces distributions sont illustrées pour divers plans situés le long de l'axe des axicons; on examine également l'effet de la présence de la monture des axicons et des principale erreurs d'alignement du système. On présente ensuite le résultat de mesures de distributions d'intensité produites en focalisant un faisceau gaussien quasi-fondamental à l'aide d'un système semblable; on constate un bon accord avec les résultats des calculs numériques. De tels calculs présentent un intérêt pour nombre d'applications : ils permettraient de prédire certaines caractéristiques importantes (par exemple, l'intensité maximale, la longueur de la ligne focale, le degré d'asymétrie) des distributions d'intensité produites par des systèmes optiques utilisant une paire d'axicons comme éléments focalisants

Resonator with variable-reflectivity output coupler

A novel resonator is proposed, in which the polarizing properties of standard grating and polarizing components (such as Brewster angle windows) inside the cavity are used to vary continuously the output coupling over a wide range of values. An analysis of the polarization properties of the modes of such a resonator is presented, and practical suggestions are made about the choice of the polarizing components. The results of an experiment involving a TEA CO2 laser are shown to verify some of the conclusions of the analysis

Boundary diffraction of an inhomogeneous wave

An exact decomposition of the diffracted field into a direct wave and a boundary diffraction wave is obtained for an incident inhomogeneous wave, namely, the complex-source-point spherical wave. Our result, in the paraxial approximation, is consistent with already published results on the diffraction of a Gaussian beam

From Gaussian beam to complex-source-point spherical wave

It is shown that the paraxial Gaussian beam becomes the complex-source-point spherical wave when all-order corrections are made according to the method of Lax, Louisell, and McKnight. Apparent contradictions between previously published first-order corrections are also discussed