666 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



A = a constant which represents the ratio of a 

 given area (m-) of the retinal image to the 

 corresponding solid angle (steradians) in the 

 visual field. 

 Since / and k are constants and since cos 6 in the 

 usual case may be assumed to be equal to unity, 

 Troland (85) proposed a unit of retinal illuminance 

 which he called the photon (now known as the tro- 

 land). The number of trolands at a given point on the 

 retina is equal to the number of nits multiplied by the 

 area of the pupil in square millimeters. 



This unit is useful in an experiment in which an 

 artificial pupil is used, but should not be used when 

 pupils of different sizes are used unless the proper 

 allowance is made for the Stiles-Crawford effect. 



Transmittance of the Eye 



The spectral transmittance of the eye from the 

 cornea to the retina as measured by Ludvigh & 

 McCarthy (59) is shown by the dots in figure 23. 

 This includes consideration of the Icsses by reflection 

 and scatter at the surfaces as well as the losses by alj- 

 sorption and scatter in the media. In considering 

 foveal vision it is necessary to pay attention to the 

 brown or yellowish spot of macular pigment covering 

 the central 14-degree region of the retina which is 

 called the macula lutea. The transmittance of the 

 macular pigment in this region according to VVald 

 (88) is given by the circles in figure 23 and the curve 

 represents the product of the two transmittances 



500 600 



WAVELENGTH, rriM 



FIG. 23. Spectral transmittance of the ocular media. The 

 circles represent the transmittance of the macular pigment, and 

 the dots the transmittance of the media from the cornea to the 

 retina. The curve represents total transmittance. [From Judd 



(44)-] 



giving the total transmittance of the ocular media. 

 The absorption in the ultraviolet region depends 

 largely upon the lens. In an aphakic eye enough ultra- 

 \iolet reaches the retina so that objects invisible to the 

 normal eye with ultraviolet illumination are easily 

 seen by the aphakic eye. 



Maxwell's spot, which can be seen when the eye 

 alternates fixation from a gray surface to a purple 

 surface of the same brightness, is probably dependent 

 upon the macular pigment. Walls & Mathews (90) 

 believe it to be a function of the distribution of differ- 

 ent kinds of photoreceptors. 



Polarization affects the amount of light reaching 

 the retina as shown by Haidinger's brushes which are 

 visible in looking at the blue .sky through a polaroid 

 filter. This polarization effect is attributed to Henle's 

 fibers (22, p. 806) which radiate from the center of 

 the fovea and connect the cones at the center of the 

 fovea with bipolar cells which are displaced toward 

 the edge of the fovea. 



Stiles-Craivjoril Effect 



Stiles & Crawford (83) have in\estigated the rela- 

 tive luminous efficiency of rays entering different parts 

 of the pupil. The results in the horizontal meridian 

 in a typical case are given in figure 24. Los.ses from 

 reflection and scattering at the refracting surfaces 

 and losses from absorption and scattering by the media 

 niav contribute to this effect but the most important 

 factor is the angle of incidence at the photoreceptors. 

 It is an effect which involves cones but not rods (82) 

 which are normally oriented. It is not affected by 

 polarization of the light (70). Phase diflFerences in two 

 beams entering different parts of the pupil (25) do 

 not affect the efficiency of the beams when thev are 

 combined again at the retina, and hence the eflfect 

 can be treated as if it were produced by a gradient 

 filter covering the pupil which has a high transmit- 

 tance at the center which tapers off at the edge. 



Stra\ Li§lit in the Eye 



There are .several sources of stray light in the eye 

 (10, 35, 36, 55, 81): fl) diffusion through the sclera and 

 iris (10, 75); flare in the optical system (55), in- 

 cluding the light reflected from the iris to the cornea 

 and thence through the pupil to the retina, and also 

 part of the light difl"usely reflected by the retina 

 which mav be reflected back toward the retina by one 



