THE IMAGE-FORMING MECHANISM OF THE EYE 



667 



4 3 2 10 12 3 4mm. 

 DISTANCE FROM THE CENTER 

 OF THE PUPIL 



FIG. 24. The Stiles-Crawford effect. Data for the horizontal 

 meridian of the right eye of B.H.C. (81). [From Fry (37).] 



of the refracting surfaces; <) scatter by the media 

 (12, 35, 36, 41, 55, 81) including the halos(22, p. 801) 

 produced by diffraction associated with the micro- 

 structure of the lens and cornea; (f) diffuse reflection 

 from the pigment epithelium, choroid and .sclera 

 (19, 34), this light stimulating the photoreceptors in 

 passing back through the retina (halation), and then 

 passing through the vitreous to the other parts of the 

 retina (after reaching some other part of the retina a 

 part of the light may be further reflected); e) fluores- 

 cence of the lens (22, p. 820) and the retina (22, 

 p. 821) when exposed to ultraviolet light; and /) bio- 

 luminescence in the photoreceptors which Judd 

 (44, 69) has proposed may cause one of the images in 

 the sequence following a flash of light. [The 'blue arcs' 

 associated with the passage of impulses along ganglion 

 cell axons across the retina are explained by some 

 writers as a form of electroluminescence, but the evi- 

 dence favors the direct electrical excitation of the 

 underlying eleinents (68).] 



In the actual use of the eyes a person is most likely 

 to run into the problem of stray light in connection 

 with the impairing effect of a peripheral glare source 

 on foveal vision. The effect of the glare source on a 

 given test object can be compared with the effect of 

 a patch of veiling luminance superimposed on the test 

 object (41, 42, 80). It is satisfactory to assume that 

 this effect is mediated by stray light (12, 35, 36, 81) 

 and hence the luminance of the veiling patch may be 

 used as a measure of the stray light. In figure 25 the 

 ratio of the veiling luminance By (nits) to the illumi- 



nance E (luxes) in the plane of the pupil produced 

 by a glare .source is plotted as a function of the angle 

 d of the glare source from the primary line of sight. 

 Stray light is especially important in interpreting 

 the results of electroretinography (11, 13, 38, 93) and 

 pupillography (39). If the eye is exposed to a small 

 bright stimulus, the electrical potentials or the 

 pupillary response produced by the millions of ele- 

 ments feebly stimulated by stray light may completely 

 mask the response of the few elements stimulated by 

 focused light. 



BLUR OF RETINAL IMAGE 



The retinal image can be treated either as a geo- 

 metrical or as a physical image. In treating the image 

 of a monochromatic point source as a geometrical 

 image, one assumes that the rays from the point source 

 which pass through the pupil are uniforinly distrib- 

 uted across the pupil. These rays can be traced to the 

 retina and the illutriinance at any part of the image 

 is proportional to the concentration of rays at that 

 point. 



In treating the retinal image as a physical image, 

 it is assumed that the light entering the eye is propa- 

 gated in the form of waves and diffraction is taken 

 into consideration. 



The concept of a blur circle produced by throwing 

 the eye out of focus and of a ijlur ellipse produced by 

 astigmatism is based upon geometrical imagery. The 



100 



05° I" 



e (DEGREES) 



FIG. 25. The equivalent veiling brightness (Bv) of a glare 

 source at various glare angles (f). [From Fry (36).] 



