648 



HANDBOOK OF PHVSUJLOGV 



NEUROPHVSIOLOGY I 



sity. This is confusing because in optics the term 

 intensity is reserved to designate the candlepower of a 

 point source. 



Furthermore, blur produces a pattern of illumi- 

 nance on the retina which is quite different from the 

 distribution of luminance in the visual field, and in 

 most cases it is the blur inherent in the image- 

 forming mechanism and not the structure of the 

 retina which limits the ability of the eye to resolve 

 fine detail. 



Stray light in the eye also presents a proljlem. Al- 

 though the stray light is feeble in comparison with 

 the focused light which is applied to a small spot on 

 the retina, it still has to be reckoned with in relating 

 the light response of the pupil and the potential of 

 the electroretinogram to the pattern of stimulation 

 applied to the retina. We are dealing not only with 

 the small number of photoreceptors responding to 

 focused light but also with the millions of photo- 

 receptors responding to stray light. 



This chapter also includes a section on entoptic 

 phenomena because they are used in various indirect 

 ways to help us understand how the eye gathers in- 

 formation. 



The study of the image-forming mechanism of the 

 eyes has a long history because as soon as man began 

 to think about himself as something separate from 

 the external world, he assumed the reality of the 

 external world and began to wonder how he could 

 .see external objects. At first he supposed that images 

 were given off by objects and transmitted into the 

 eye. He reasoned that these images must be reduced 

 in size in order to get through the pupil. The discovery 

 of the small images reflected by the cornea led to the 

 belief that these images are responsible for \'ision, and 

 the lens and not the retina was assumed to be the 

 structure assigned to relay the images to the brain. 

 This view lasted for centuries. About the beginning of 

 the seventeenth century, Kepler (79, p. 116) dis- 

 covered and described how an image is formed by a 

 refracting surface. He then applied his concepts to 

 the eye to show how the refracting mechanism of 

 the eye must form an upside-down picture on the 

 retina. The pinhole camera which was invented about 

 the same time helped to demonstrate how an upside- 

 down image could be formed, and finally Scheiner 

 (79, p. 116) demonstrated the upside-down image 

 on the back of an excised eye. 



.•\ijout this time attention was turned away from 

 the nature of the image on the retina to the mecha- 

 nism of accommodation by which the eye can change 

 its focus. From the time of Kepler to that of Young 



C79> P- 158) various mechanisms were proposed in- 

 cluding change in length of the eye, change in the 

 curvature of the cornea, change in the position and 

 shape of the lens and change in the size of the pupil. 

 Young (86, p. 201; 79, p. 158) with a series of bril- 

 liant experiments at the beginning of the nineteenth 

 century showed that the lens provides the basis for ac- 

 commodation. Since then steady progress has been 

 made in understanding the various aspects of image 

 formation by the eye. 



Helmholtz (79) has presented at the end of each of 

 his chapters an historical summary and a biljli- 

 ography which is useful to those interested in the 

 early history of the subject. There are other general 

 references that pertain to the early history (74, 77, 78). 



IMAGE FORMATION 



Giillstriind' s Schematit Eve and Its Refracting Mechanism 



In demonstrating the principles of image forma- 

 tion by the eye, it is customary to substitute for an 



FIG. I. Gullstrand's schematic eye, in which the dimen- 

 sions and indices are as follows: 



mm 



Thickness of cornea 0.5 



Displacement of front surface of lens behind front 



surface of cornea 3.6 



Displacement of nucleus from front surfaceof lens . . 0.546 



Thickness of nucleus 2.419 



Thickness of lens 3.6 



Index of refraction of cornea i . 376 



Index of aqueous and vitreous i .336 



Index of lens cortex 1 .386 



Index of lens nucleus i .406 



Radius of front surface of cornea 7.7 



Radius of back surface of cornea 6.8 



Radius of front surface of lens lO.o 



Radius of front surface of nucleus 7 •9' ' 



Radius of back surface of nucleus ~5-7^ 



Radius of back surface of lens —6.0 



