Peter McIntyre

The spectral sensitivities of identified receptors and the function of retinal tiering in the principal eyes of a jumping spider

Summary1.The functional organisation of the central retina of the anterior median (AM) eyes of a jumping spider,Plexippus (Salticidae) is examined by anatomical, electrophysiological and optical methods. A model of the eye is derived from the data.2.The anatomy of the AM eye is similar to that of salticid eyes described by Land (1969a) and Williams and McIntyre (1980). There are four tiers of receptors of which only the most proximal (Layer I) is a regular mosaic with rhabdoms designed to have light-guide properties. The receptor population of Layer I is homogeneous, whereas in Layers II–IV more than one receptor type can be considered to contribute to each layer.3.Intracellular recordings from AM photoreceptors reveal only two spectral classes: green cells with peak responses at ca. 520 nm, and ultraviolet (UV) cells with peak responses at ca. 360 nm. ERGs from intact retinae exhibit similar peaks. Spectral sensitivities from pooled intracellular recordings from green cells and ERGs correspond reasonably closely. The comparison does not, therefore, support the possibility that the retina contains receptors with peak responses at longer wavelengths, although it does not exclude it.4.Spectrally characterised cells were marked by the injection of Lucifer Yellow. From the results of 13 successful injections, (a) peripheral Layer I and peripheral and central Layer II cells are green receptors; (b) Layer IV cells are UV receptors. Central Layer I and Layer III receptors were not marked.5.The chromatic aberration, focal length and other optical parameters of the corneal lens of the AM eye were measured directly. The lens functions essentially as a single-surface lens of refractive index 1.40, and, together with the curved interface between the anterior chamber of the eye and the receptor matrix, forms a telephoto system.6.The spacing between receptor Layers I and IV is matched to the chromatic aberration of the eye; if green light from an object in front of the spider is focused on Layer I, UV light will be focused on Layer IV (and Layer III).7.The distal ends of Layer I receptors form a staircase, those lying laterally being closer to Layer II than those lying medially. This staircase enables the spider to receive in-focus images from objects at distances between ca. 3 cm — ∞ in front of it. It is suggested that the scanning movements of the retinae described by Land (1969b) serve to sweep an image across the staircase so that it will be in focus on some part of Layer I, provided that the object is within that range of distances.8.Retinal tiering (including the staircase of Layer I) compensates both for the chromatic aberration of the dioptrics of the eye and for its inability to accommodate.

Power transfer between optical fibers

Previous papers have treated power transfer between HE11, TE01, and TM01 modes propagating on identical cylindrical fibers. Here we extend the theory to include power transfer between modes of any order propagating on uniform circular fibers of different radii and dielectric constant. A simple analytical expression for the coupling coefficient is derived. The error in using the decoupled two-mode form of the coupled-mode equations is determined. Examples are given to illustrate the extension of the two-fiber results to arrays of fibers with different properties. All results are presented in a dimensionless form applicable to circularly cylindrical fibers of arbitrary physical parameters.

Electrophysiological analysis of fly retina

Summary1.Intracellular recordings have been made from over 100 central retinula cells (R7 and R8) in the fliesCalliphora stygia (wild type),Calliphora erythrocephala (chalky) andMusca domestica (white).2.Spectral sensitivities determined with axial and non-axial illumination (Figs. 2, 3, 7, 9), polarisation sensitivities and their dependence upon stimulus wavelength (Table 2, Figs. 5, 6), and the effects of intense chromatic adaptation (Fig. 4) are described.3.All retinula cells R7 investigated had a major peak of spectral sensitivity in the ultraviolet. One class with a peak at ca. 340 nm and less than 10% sensitivity remaining at wavelengths longer than 400 nm, are termed UV cells. Another class with a peak at ca. 360 nm and a long tail of sensitivity (>10%) extending to 500 nm, are termed UT cells (Fig. 2).4.The ratio of UT cells to UV cells was approximately 7∶3, this is similar to the ratio between the two classes of distal central rhabdomeres, seven yellow (7y) and seven pale (7p) (after Kirschfeld and Franceschini, 1977) (Table 1).5.These results, in combination with recent microspectrophotometrical data (Kirschfeld et al., 1978; Kirschfeld, 1979) support the following interpretations:a)UV cells possess 7p rhabdomeres which contain an ultraviolet-absorbing rhodopsin that can be reversibly photoisomerised to a blue-absorbing metarhodopsin.b)UT cells possess 7y rhabdomeres and contain a different photopigment system to that found in UV (=7p) cells (Kirschfeld, 1979). Possibly this consists of a blue-absorbing rhodopsin and an ultraviolet sensitising pigment. The blue-absorbingphotostable pigment found in 7y rhabdomeres (Kirschfeld et al., 1978) has a screening function and modifies both spectral and polarisation sensitivity in UT cells.6.The R8 cells encountered probably all had rhabdomeres lying beneath 7y rhabdomeres; their spectral sensitivity could be accurately modelled by the screening action of the pigments present in 7y rhabdomeres upon a visual pigment with a peak absorbance at ca. 520 nm (Fig. 9).

Light propagation in twisted anisotropic media: Application to photoreceptors

The propagation of light through a slowly twisting anisotropic medium is described by a coupled-mode theory; expressions are derived for the electric field for the case of a birefringent dichroic medium with a constant rate of twist. The method provides a simple and intuitive means for determing the effect of twisting on the linear birefringence and dichroic absorption of the medium, particularly when the light is initially linearly polarized. The theory is well suited to the analysis of light absorption in twisting insect photoreceptors, such as found in bees and ants. We provide full expressions and useful approximations for polarization sensitivity and the initial direction of polarization to give maximum absorption for several types of photoreceptors.

Image quality and acceptance angle in a reflecting superposition eye

Summary1.Intracellular recordings of acceptance angles were made at different adaptation states and at different times of day, from retinula cells in the central region of the reflecting superposition eye of the freshwater crayfish,Cherax destructor (Clark 1936). A number of cells were held for longer than 24 h.2.The acceptance angle is greater in the dark-adapted (DA) state than in the light-adapted (LA) state, but this difference is significantly smaller at night than during the day. Mean values were 5 ° (LA day and night), 19 ° (DA day) and 13 ° (DA night).3.The anatomy was studied with proper attention to the time of day. Light microscopy shows that there is an increase of 10–15% in the clear-zone width in the DA state at night compared to the other states. This is achieved by a proximal movement of the rhabdom layer.4.The position of the distal screening pigment depends on the ambient light level. It is around the distal cones in the dark and moves into the clear zone on light-adaptation. The proximal screening pigment, however, attains only the first stage of dark-adaptation (Frixione et al. 1979) during the day. Complete dark-adaptation (movement below the basement membrane) occurs only at night. The distal reflecting pigment remains around the cones at all times, and the proximal reflecting pigment forms a cup around the base of the rhabdom during both day and night.5.The smaller acceptance angle in the DA state at night, compared to that of the DA day state, indicates that the receptors lie closer to the best superposition focus in this state, when the animal is in air.6.The effect of the change in clear-zone width and the performance of the eye in air and under-water are examined using a model of the eye.

Absorption properties of a photostable pigment (P456) in rhabdomere 7 of the fly

SummaryOne photoreceptor type (R7y) in the compound eye of fly (Musca) contains, besides the visual pigment, a photostable pigment (most probably a carotene) with maximal absorption in the blue spectral range. The extinction and dichroism due to this pigment are determined, taking into account waveguide properties, birefringence, anomalous dispersion and possible twisting of the rhabdomeres. The concentration of this pigment, if it is a carotene, is rather high: there are 7–10 molecules per rhodopsin molecule.

Crosstalk between light pipes

Using mode theory, we calculate the crosstalk between overmoded optical fibers or light pipes when one fiber is illuminated by a focused collimated beam. The presence of many modes allows the summation of crosstalk power between individual modes to be converted to an integral expression. The result is applied to two parallel, identical fibers, a hexagonal array of fibers, lossy fibers, and fibers of unequal diameter. In practice, the light pipes must be nearly identical and touching for significant crosstalk.

Power transfer between nonparallel and tapered optical fibers

Coupled-mode theory is used to find the power transfer between neighboring fibers when the coupling is dependent on the position along the fibers, as in nonparallel or tapered fibers. If the coupling coefficient is replaced by an averaged coupling coefficient, the usual coupled-mode solutions are found to apply. Curves of the averaged coupling coefficient are presented in a length-independent form for HE11 modes on nonparallel and tapered fibers of arbitrary physical parameters.

Transmission of light through a twisted nematic liquid-crystal layer

A coupled-mode theory is used to find simple exact expressions for the transmission of light through a twisted nematic liquid-crystal layer. The thickness of the layer for zero transmission is shown to depend on the total twist angle. A previous theoretical model for tuning a liquid-crystal cell is shown to be in error and alternatives are suggested.

Polarisation Sensitivity of Twisted Fused Rhabdoms

MENZEL (this volume) has discussed the morphology associated with the twisting of the ant rhabdom. Here we consider how the twisting affects the absolute and polarisation sensitivities of the individual retinula cells. The analysis follows from a straight-forward extension of the theory presented in the appendix of SNYDER (1973).