338 research outputs found
Hyperopia is predominantly axial in nature
No full textPURPOSE: Myopia has been found to be predominantly axial in nature, i.e. myopic eyes have longer than normal axial lengths, with corneal radius variations having only a small influence on the magnitude of the refractive error. In this study we assess whether a similar relationship exists for hyperopia. METHODS: Biometric data were collected on 57 subjects with either emmetropic or hyperopic refractive errors ranging in magnitude from -0.37 D to +17.25 D. Our main analysis concentrated on subjects with less than +10 D of hyperopia (group 1, n = 53), as subjects with +10 D of hyperopia or more (group 2, n = 4) exhibited marked differences in their biometric characteristics. RESULTS: Analysis of group 1 data revealed a significant relationship (r2 = 0.611, p = 0.0001) between the degree of hyperopia and the measured axial lengths. A weak but statistically significant relationship (r2 = 0.128, p = 0.009) was also found between mean corneal radius measures and mean spherical refractive errors, with the mean corneal radius flattening with increasing hyperopia. In group 2, three of the four subjects exhibited much steeper corneal characteristics than predicted from the group 1 data. CONCLUSIONS: Our results suggest that hyperopia, like myopia, is predominantly axial in nature, although the corneal radius also plays a role in determining refractive error magnitude. These results have implications for refractive surgery and visual performance in hyperopic eyes
Measuring contrast sensitivity with inappropriate optical correction
No full textSpatial frequency-selective minima (notches) in the contrast sensitivity function (CSF) because of defocus can mimic those that occur with ocular disease. We examined the influence of measurement conditions on CSF shape in simulated clinical testing. CSF notches occurred with almost all levels of defocus for all subjects. Multiple notches were found under some conditions. Notches were found with defocus as small as 0.50 D. Effects of induced astigmatism depended on the orientation of the target. Notches were apparent in defocus conditions after stimulus size and room illuminance were modified and when subjects had insufficient accommodation to compensate for hypermetropic defocus. The equivalent of notches was not noted with the Pelli–Robson chart. As defocus-induced CSF notches may be mistaken for functional loss, careful refractive correction\ud
should be conducted prior to clinical or experimental CSF measurement, even at low spatial frequencies
Effects of defocus and pupil size on human contrast sensitivity
No full textDefocus lowers the contrast sensitivity function (CSF), producing a complex function with local dips and peaks. Previously, we were able to predict the shape of the CSF with large pupils from measured transverse aberrations with hypermetropic defocus but not with myopic defocus (Atchison et al., 1998c, J. Opt. Soc. Am. A. 15, 2536). As there is no reason that myopic defocus should be more difficult to predict than hypermetropic defocus, we modified the procedure to try to improve CSF predictions with myopic defocus. Also, we extended the study to consider a range of pupil sizes. CSFs were measured for three subjects at three defocus levels (in-focus, -2D and +2D) and three pupil sizes (2 mm, 4 mm and 6 mm). Using a diffraction optics model, transverse aberration measures and in-focus CSF measures, we predicted the defocused CSFs. The predicted defocused CSFs were lower than the in-focus CSF as expected, and had complex shapes that varied with defocus and pupil size and between subjects. While a few predictions were poor, generally, the overall magnitude and shape of the defocused CSFs were well predicted and similarly so for myopic and hypermetropic defocus. Some further improvements in technique are indicated
Dynamic accommodation response in the presence\ud of astigmatism
No full textIt has been suggested that in the presence of astigmatism some individuals make cyclic changes in focus over the astigmatic interval to obtain better visual performance. The aim in the present study was to identify such cyclic accommodative behavior and to characterize the variability of the response in the presence of astigmatism.\ud
The dynamic accommodation response in the presence of induced astigmatism was recorded objectively with an infrared optometer in seven young adults. Astigmatism led directly to increased accommodative variability in certain individuals. In two of seven participants there was evidence for aperiodic cyclic accommodative responses between different portions of the astigmatic interval. However, the amplitude of these tracking\ud
responses was much smaller than the astigmatic interval
Influence of Stiles-Crawford apodization on visual acuity
No full textThe Stiles–Crawford effect (SCE) of the first kind has often been considered to be important to spatial visual performance in that it ameliorates the influence of defocus and aberrations. We investigated the influence of SCE apodization on visual acuity as a function of defocus (out to 62 D) in four subjects. We used optical\ud
filters, conjugate with the eye’s entrance pupil, that neutralized or doubled the existing SCE. With an illiterate-E task, the influence of the SCE was more noticeable for myopic defocus than for hypermetropic defocus, was generally more noticeable for high-contrast than for low-contrast letters, and increased with increase\ud
in pupil size. The greatest influence on visual acuity of neutralizing the SCE, across the subjects and range of conditions, was deterioration of 0.06 (4-mm pupil), 0.16 (6-mm pupil), and 0.29 log unit (7.6-mm pupil)
Ceum air Cheum: Cruinneachadh de dhàintean nas fhaide = Step by Step: A Book of Longer Poems
No full textThis is Christopher Whyte's sixth collection. Ceum air Cheum features 12 longer poems. Its European range, modernity of tone and challenging subject matter mark this book out as a major event in Gaelic poetry. Facing translations into English are by Niall O’Gallagher and the author
Alexandria: A City and Myth
No full textSynopsis
Alexandria was one of the most important cities of the ancient world, with achievements in the arts, sciences and religion. For the first time the author seeks to understand the wider picture, the longer period of evolution as a city, as both an urban concept and a literary and historical ideal. He does this by bringing together the disciplines of archaeology (including his own recent fieldwork), anthropology, history, geography, oral history, art and literature. As a result Alexandria is seen as a unique example of African urbanism, an Egyptian city facing the wider Mediterranean world, which became an archetype for social, religious and cultural cosmopolitanism. A work for undergraduates and postgraduates in the disciplines of classical and Egyptian archaeology, historical geography, art history, oriental studies and general history
Imposed retinal image size changes--do they provide a cue to the sign of lens-induced defocus in chick?
No full textBACKGROUND: Young chicks can adjust their eye growth to compensate for both imposed hyperopia and myopia (using negative and positive spectacle lenses); the rate of eye elongation increases in the former and slows in the latter case. This emmetropizing behavior implies that the eye can distinguish the sign and magnitude of defocus, although the identity of the cue(s) involved is unknown. As the spectacle lenses used in these studies generally introduce significant retinal image size differences that are in opposite directions for negative and positive lenses (minification vs. magnification), we asked whether retinal image size might provide the required sign information. METHODS: This question was addressed by manipulating retinal image size while keeping lens power constant. We also investigated the effect of eliminating other potential cues, accommodation and chromatic aberration, under these conditions. Three negative "size" lenses of approximately -11 D optical power were used, with 2 of the lenses producing magnification rather than minification as typical of negative lenses (i.e. +1.9% and +6.9% compared to -2.9%). The lenses were fitted monocularly to 7-day-old chicks, which were subsequently measured at 9 and 11 days of age (refractive error and axial dimensions). The same lens-wearing schedule was applied to two other groups of chicks that had monocular ciliary nerve section surgery to prevent accommodation 2 days posthatching; one of these groups was reared under monochromatic yellow light instead of white light. RESULTS: Near-perfect refractive compensation was seen by the end of the treatment period with all three lenses, for all three treatment groups, and there was also little difference in the rate of compensation among the various groups. In all cases, the typical responses of axial (mainly vitreous chamber) elongation and myopia were observed. CONCLUSIONS: That manipulations to retinal image size, which either decrease or reverse the usual effects of negative lenses, did not disrupt compensation to the imposed hyperopic defocus, even in the absence of accommodation and chromatic aberration cues, argues against imposed retinal image size changes being the directional cue to defocus in experimental emmetropization
Deleuze and the author
No full textThis thesis argues that Gilles Deleuze, as philosopher, reader, and critic, recognised the
central importance of a defined authorial subjectivity, closely associated with a
philosophical or intellectual project, and that his analyses of philosophy, literature, visual art
and cinema were shaped and determined by his recognition of that authority. In this
respect, my reading challenges those critics who find in the work of Deleuze an assault on
‘author-centric’ interpretations of texts, and more generally on the concept of a unified self,
and which uphold experimentation on the part of the reader or critic rather than
interpretation. I argue that Deleuze has a coherent and meaningful conception of an author
as a consciousness which persists through time, learns, plans and makes projects,
differentiates itself from the work of other authors, is inspired and creative, takes positions
in relation to the inheritance of artistic and philosophical traditions, and which is capable of
entering into collaboration with others.
Through close reading of Deleuze’s texts, I demonstrate that he consistently relies on the
authorial function to impose unity and coherence on the distinctive - and often remarkable
- body of work of an individual theorist or practitioner. I argue that the historical, political
and social situation of an author is of great importance to the analysis of a text. Finally,
unlike Roland Barthes or other critics invested in the ‘death’ or displacement of the author, I
argue that Deleuze considers the competing interpretations of a text advanced by the
reader or spectator to be of little or no importance
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