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The gradient index lens of the eye: an opto-biological synchrony
No full textThe refractive power of a lens is determined largely by its surface curvatures and the refractive index of its medium. These properties can also be used to control the sharpness of focus and hence the image quality. One of the most effective ways of doing this is with a gradient index. Eye lenses of all species, thus far, measured, are gradient index (GRIN) structures. The index gradation is one that increases from the periphery of the lens to its centre but the steepness of the gradient and the magnitudes of the refractive index vary so that the optics of the lens accords with visual demands. The structural proteins, the crystallins, which create the index gradient, also vary from species to species, in type and relative distribution across the tissue. The crystallin classes do not contribute equally to the refractive index, and this may be related to their structure and amino acid content. This article compares GRIN forms in eye lenses of varying species, the relevance of these forms to visual requirements, and the relationship between refractive index and the structural proteins. Consideration is given to the dynamics of a living lens, potential variations in the GRIN form with physiological changes and the possible link between discontinuities in the gradient and growth. Finally, the property of birefringence and the characteristic polarisation patterns seen in highly ordered crystals that have also been observed in specially prepared eye lenses are described and discussed
New insights into structural changes of lens proteins
No full textX-ray scattering techniques were used to study the effects of heating on whole eye lens and α-crystallin gels. The temperature range used was from 20 to 70°C. The position of single X-ray reflection seen in whole lens was unchanged in the temperature range 20 to 45°C, with a continuous spacing of 152 Å. However, at 50°C the spacing increased from 152 Å to 165 Å. An interpretation of these results is that in eye lens, α-crystallin is protecting other lens proteins from super-aggregation up to 50°C. In α-crystallin gels a moderate increase in both the spacing and intensity of the reflection was observed from 20 to 45°C, followed by a dramatic increase from 45 to 70°C. Over the whole temperature range the spacing changed from 138 Å at 70°C to 195 Å at 70°. After eleven hours of cooling, this effect was found to be irreversible
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Order-Disorder Phenomena in Polyelectrolyte Tissues
Get PDFThree polyelectrolyte-based tissues, muscle, cornea and lens have been investigated using microelectrode and low angle X-ray diffraction techniques. It has been found that with increasing temperature the net fixed charge in the A- and I-bands in muscle falls, in both the relaxed and rigor states. In the rigor state a definite temperature transition between 27.5 - 30 °C has been established. The sharpness of the off meridional peaks of the 365 Å actin layer line of rigor muscle increases with increasing temperature at low ionic strength. In relaxed muscle previous reports show that the myosin layer-lines disappear with decreasing temperature. We have shown that.the net charge decreases with increasing temperature, as these two phenomena occur within the same temperature range, it is probable that they are connected. All these effects are explained in terms of ion binding models.
Previous reports have shown that there is an anomalous negative temperature coefficient of Donnan swelling theory in cornea. We have shown that the net fixed charge decreases linearly with increasing temperature in corneal stroma. This is explained in terms of an ion binding model, which accounts for the negative temperature coefficient.
In calf lens no change in the measured Donnan potentials is observed below the critical temperature for the onset of the cold cataract phenomenon. This is probably due to the low concentration of the cryoprotein responsible for this effect.
Micromolar concentrations of ATP and PPi have been shown to decrease the fixed negative charge on the myosin molecule in previous work. We have shown that in muscle at zero overlap the lattice spacing and X-ray equatorial intensities are at a maximum at micromolar concentrations of PPi. This effect is interpreted in terms of a change of the electrical regime within the muscle lattice
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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