PHOTORECEPTION 201 



fraction, light coming through a small circular aperture produces an 

 Airy pattern, a central point of maximum intensity with two concen- 

 tric dark rings alternating with light rings. If two point sources are to 

 be resolved they must be far enough separated so that the bright image 

 centre of one falls no closer than within the first dark ring of the Airy 

 pattern of the other. Miiller (1826) had said that an apposition omma- 

 tidium was sensitive only to light from a point on or near its axis. It is 

 maximally sensitive on its axis and minimally sensitive away from its 

 axis, so the diameter determines how close two points can be if one is to 

 stimulate maximally and the other minimally. In the honeybee, if the 

 eye resolves points separated by twice the minimal ommatidial angle 

 the performance of the ommatidia has approached the theoretical 

 limit set by diffraction. 



It is by no means certain, however, that the ommatidium is the 

 optical unit of the eye. Refined techniques which permit illumination 

 of single rhabdomers (in Diptera) have prompted the suggestion that 

 it is the retinal cell which is the functional unit of the eye (de Vries, 

 1956; de Vries and Kuiper, 1958; see also Burkhardt and Wendler, 

 1960; Antrum and Burkhardt, 1961). By cutting transversely through 

 an ommatidium in the region of the rhabdom and examining the cut 

 end of the rhabdom as light is transmitted through the crystalline cone 

 it can be seen that each rhabdom 'looks' at a different part of the visual 

 field. Each visual field has a diameter of about 4 degrees, and the fields 

 of neighbouringcells overlap so that the centre of one field lies approxi- 

 mately at the edge of the neighbouring field. The field of the whole 

 ommatidium is about 8 degrees, and it overlaps with that of its 

 neighbours. As with the entire ommatidium when it is analysed as a 

 unit, the field of the individual cell is controlled by the diameter of the 

 rhabdom and the diffraction of Hght. If a point source of light being 

 viewed at the cut end of a rhabdomer is displaced more than 2 

 degrees from the optical axis of the retinal cell, it shines again, but 

 more weakly, by reason of the diffracted hght outside of the first dark 

 ring of the Airy pattern. Once a beam has reached a rhabdomer it will, 

 unless it deviates too far from the optical axis, be reflected repeatedly 

 at the boundaries of the rhabdomer, since it has a higher refractive 

 index than surrounding tissues. In this manner the rhabdomer acts as 

 a wave-guide, and the cell captures light (see also Demoll, 1917). 

 Since the internal reflexion decreases as the angle between the incident 

 beam and the axis of the cell increases, there is a directional sensitivity 

 in these cells, the Stiles-Crawford effect of human vision (de Vries 

 and Kuiper, 1958). 



