[An investigation on the freshwater crab as the host of Paragonimus in Hunan province]

Through an investigation of the freshwater crabs in 39 counties of Hunan province, 20 species of fresh water crabs were found as the intermediate hosts of Paragonimus. They belong to three genus and two families, in which 10 are new species (including 4 new subspecies). Besides, the natural infection with lung fluke metacercariae was also observed in some of the fresh water crabs.

Cytogenetic effects of low-level exposure to toluene, xylene, and their mixture on human blood lymphocytes

The present study was undertaken to investigate, both in vitro and in vivo, the genotoxic potential of short-term low-level exposure to toluene, xylene, and their mixture, for which information is limited at the present time. Five adult healthy white men were exposed for 7 consecutive hours per day over 3 consecutive days to 50 ppm toluene and 40 ppm xylene either alone or in combination in a controlled exposure chamber. Such an exposure was repeated three times at intervals of 2 weeks. Blood samples were taken before and after the termination of such exposure. Three different cytogenetic end-points were evaluated using peripheral blood lymphocytes: number of sister chromatid exchanges (SCEs), cell cycle delay, and cell mortality. No significant effects on SCEs, cell cycle delay, and cell mortality were observed following such exposure to toluene or xylene or their mixture. Similarly, exposure of human blood lymphocytes in vitro to either toluene (0-2.5 mM) or xylene (0-2 mM) or their mixture for 72 h did not result in any significant cytogenetic effects at lower concentrations, while at higher concentrations, only cell mortality was found to be significantly affected. Thus our present study indicates that simultaneous exposure to low levels (within the admissible limits) of toluene, xylene, or their mixture for a short period does not pose any potential mutagenic threat to humans.

Effect of ocular chromatic aberration on monocular visual performance

This brief review outlines the theory of ocular chromatic aberration and describes the three primary forms in which the aberration appears: chromatic difference of focus, chromatic difference of magnification, and chromatic difference of position. Our central theme is that all three aspects of chromatic aberration have as their common basis the chromatic dispersion of light. The magnitude of each form of the aberration is related to the others by simple linear formulas in which a key parameter is the location of the pupil relative to the nodal point of the eye. The way in which retinal image quality is affected by chromatic aberration is described and we assess the impact of the aberration on visual performance.

Achromatizing the human eye

Ocular chromatic dispersion manifests itself as wavelength-dependent image planes, image sizes, and image positions, and it has been suggested that ocular chromatic aberration is the most important of the eye's optical aberrations. Most attempts to correct for the eye's chromatic aberration (achromatize the human eye) have concentrated on correcting the wavelength-dependent image planes or chromatic difference of refractive error (CDRx). There are two optical techniques that correct for CDRx (special achromatizing lenses and multiple channel display systems) by making the ocular image planes of all wavelengths coincident. A different approach simply avoids the effects of ocular CDRx by using small pupils which effectively make all images diffraction-limited irrespective of wavelength-dependent differences in image planes. Theoretical and experimental evidence shows that achromatizing lenses provide an accurate correction for CDRx. In spite of the pre-eminence of chromatic aberrations, and the effectiveness of the corrections, no obvious improvements in vision accompany correction. We show that loss of retinal image quality due to CDRx may be subthreshold (less than ocular depth of focus). We also show that achromatizing methods can introduce their own chromatic aberrations that can easily exceed those present in the uncorrected eye. The precise location of the eye with respect to the achromatizing device determines the amount of these additional aberrations. Therefore, in order to achromatize the eye effectively, careful control of eye position is essential.

Relation between the chromatic difference of refraction and the chromatic difference of magnification for the reduced eye

We provide a simple model for calculating the chromatic difference of magnification for the human eye. Spectral differences in image size are proportional to the eye's longitudinal chromatic aberration and the axial distance between the entrance pupil and nodal point. We verify that the model provides magnification estimates equal to previously published predictions, and we show the significant increase in this aberration produced by experimental use of artificial pupils.

Achromatizing the human eye: the problem of chromatic parallax

Attempts to correct the chromatic difference of focus of the human eye will introduce unwanted chromatic parallax if the eye is misaligned with the optical axis of the achromatizing system. Using geometrical optics, we show that the amount of parallax is approximately proportional to the amount of misalignment of the eye, with the constant of proportionality equal to the eye's chromatic difference of refractive error. This prediction was confirmed by the experimental determination of chromatic parallax for two commercially available achromatizing lenses. On the basis of these results, we calculated that anticipated improvements in the polychromatic modulation transfer function of the eye offered by achromatizing lenses will be canceled by approximately 0.4 mm of the misalignment between the lens and the eye. Our prediction that further misalignment would severely reduce image quality of the achromatized eye was verified by psychophysical measurements of contrast sensitivity.

Interocular differences in transverse chromatic aberration determine chromostereopsis for small pupils

Chromostereopsis has been attributed previously to interocular differences in foveal transverse chromatic aberration (TCA). We tested this hypothesis by measuring chromostereopsis as a function of the separation of small artificial pupils. We also measured the monocular transverse chromatic aberration under the same conditions. Our results show that chromostereopsis with small pupils can be precisely accounted for by the interocular difference in monocular transverse chromatic aberration. This relationship is closely predicted by a simple water eye model.

Isolation and characterization of a molecular cDNA clone of a human mRNA from interferon-treated cells encoding nucleolar protein B23, numatrin

A cDNA clone encoding human nucleolar phosphoprotein B23, numatrin, was isolated from a library prepared with mRNA from human U cells. The complete nucleotide sequence was determined; it revealed a single open reading frame of 294 amino acids that included four in-frame AUG codons as potential sites of translation initiation. Comparison of the human B23 nucleotide sequence with the rat and mouse B23 sequences revealed 91% homology (hum:rat, and hum:mus) in the coding region; the predicted B23 proteins displayed 94% amino acid identity. Northern gel blot analysis revealed a single B23 mRNA species of approximately 1.5 kb. The level of B23 mRNA in U cells was not detectably altered by treatment with either alpha or gamma interferon. Southern gel blot analysis revealed polymorphism within the human B23 gene structure, and suggested the presence of multiple B23 genes and/or extensive splicing of B23 RNA transcripts.

Does the chromatic aberration of the eye vary with age?

The longitudinal chromatic aberration of the eye has been reported to decline with age. Using three different methods, we have measured the aberration in a group of young subjects (27-33 years old) and a group of older subjects (48-72 years old). In two of the methods we used a Badal optometer, either with or without an achromatizing lens incorporated, to examine the effect of wavelength on refractive error. In the third method we used a vernier-alignment apparatus to assess chromatic dispersion directly. None of the results of the experiments performed revealed any difference in aberration between the groups. Furthermore, a linear regression of aberration against age showed no relationship between these variables. We conclude that, for human adults, the magnitude of chromatic aberration is independent of age.