THE IMAGE-FORMING MECHANISM OF THE EVE 



669 



on 



POINT 



LINE 



BORDER 



FIG. 30. Distributions of illuminance in the images of a point, 

 a line and a border, representing the same degree of blur. 



retina may be calculated for any pattern. The index 

 of blur <j) proposed by Fry & Cobb (37, p. 33) pro- 

 vides a method of specifying the amount of blur 

 regardless of how it is caused. Figure 30 illustrates the 

 distribution of retinal luminance across the images of 

 a point, a line and a border. Although 4> may be de- 

 fined in terms of any one of these images, its meaning 

 is best comprehended in the case of a line. It repre- 

 sents the ratio of the area under the curve to the height 

 of the central ordinate. <t> has the advantage that for 

 any condition of observation it can be measured ex- 

 perimentally without analyzing the factors that 

 contribute to it. 



ENTOPTIC PHENOMENA 



Entoptic phenomena include the shadows on the 

 retina of opaque structures inside the eye and the 



nonuniformities in the illumination of the retina pro- 

 duced b\ nonuniformities of index and surface 

 cur\ature. 



A white surface \iewed through a pinhole held 

 near the primary focal point produces a shadow of the 

 iris on the retina. The irregularities in the margin are 

 clearly visible and changes in the size of the pupil 

 can be observed directly. The Broca pupillometer 

 (57, p. 237) is based on this method of viewing the 

 pupil. This method of viewing also makes visible spots 

 and folds in the cornea, star figures and incipient 

 cataracts in the lens, and opacities in the \itreous body 

 which give rise to the muscae volitantes. One can also 

 see images of the blood corpuscles in the retinal capil- 

 laries as white spots. If the pinhsle is oscillated back 

 and forth, one can observe shadows of the large and 

 also the minute blood \essels. The larger vessels form 

 a branched tree known as the Purkinje figure. 



This Purkinje figure formed by the blood vessels 

 can also be \iewed !:>> illuminating a spot on the 

 sclera or by forming a small bright image on the 

 peripheral retina. By using two point sources, two 

 shadows can be produced whose angular separation 

 at the second nodal point can be measured. From this 

 one can determine the distance of the vessel in front 

 of the photosensitive elements. 



Since the vessels move with the photosensitive ele- 

 ments, their shadows are not affected by the 

 micronystagmoid movements of the eye; this pro- 

 \ides a means of studying adaptation without in- 

 vohing micronvstagmoid mosements. 



REFERENCES 



1. Allen, M. J. Am. J. Opiom. 26: 279, 1949. 



2. Allen, M.J. Am. J. Opiam. 27: 287, 1950. 



3. Allen, M. J. Am. J. OpIom. 30: 78, 1953. 



4. Allen, M.J. Am. J. Oplom. 30: 393, 1953. 



5. .Allen, M. J. Am. J. Oplom. 31 : 297, 1954. 



6. .Allen, M. J. Am. J. Oplom. 32: 422, 1955. 



7. Allen, M. J. Am. J. Oplom. 33: 201, 1956. 



8. Alpern, M. Am. J. Oplom. 27: 491, 1950. 



9. Ames, A. Jr. and C. A. Proctor. J. Opt. Soc. Am. 5: 22, 

 1921. 



10. Bartlev, S. H. Vision, A Sludy nf lis Basis. Neu- York: 

 Van Nostrand, 1941, p. 57. 



1 1. Bovnton, R. M, J. Dpi. Soc. Am. 43: 442, 1953. 



12. Bovnton, R M., J. M. Enoch and W. R. Bush. J. Opi. 

 Soc. Am. 44:879, 1954. 



13. Bovnton, R. M. and L. .A. Riggs. J. E.xper. Psychol. 42: 



■^'7. I95I- 



14. Campbell, F. W. Bnl. orlhopl. J. 1 1 : 13, 1954. 

 X5. Campbell, F. W.J. Physiol. 133: 31, 1956. 



16. Chin, N. B. and R. E. Horn. J. OpI. Soc. Am. 46: 60, 

 '956. 



17. Cobb, P. W. Am. J. Physiol. 36: 335, 1915. 



18. Davis, G.J. and F. W. Jobe. Am. J. Oplom. 34: 16, 1957. 



19. Di Francia, G. T. and L. Ronchi. J. OpI. Soc. Am. 42: 

 782, 1952. 



20. DoNDERS, F. C. .Accommodalion and Rejraction oj Ihe Eye, 

 translated by \V. D. Moore. London: New Sydenham 

 Society, 1864, p. 204. 



21. DuANE, A. Am. J. Ophlh. 8: 196, 1925. 



22. Duke-Elder, \V. S. Texl-book of Ophlhalmolog\. St. Louis: 

 Mosby, 1934, vol. I. 



23. Duke-Elder, \V. S. Texl-book oj Ophlhalmology. St. Louis: 

 Mosby, 1949, vol. IV. 



24. Emsley, H. H. Visual Opiics (5th ed.). London: Hatton, 

 1952, vol. \, p. 257. 



25. Enoch, J. M. J. Opt. Soc. Am. 48: 392, 1958. 



26. Finch AM, E. F. 1 he Mechanism of Accommodation. London: 

 Pullman, 1937. 



