CHAPTER XXVII 



The image-forming mechanism of the eye 



GLENN A. FRY | School of Optometry, The Ohio State University, Colimhus, Ohio 



CHAPTER CONTENTS 



Image Formation 



GuUstrand's Schematic Eye and Its Refracting Mechanism 



Formation of Image by Refracting Mechanism 



Refracting Power of the Eye 



Helmholtz Schematic Eye 



Reduced Eye 



Role of Pupil in Image Formation 



Chief Rays 



Blur Circles in Eye Free from Astigmatism 



Astigmatism 



Lines of Sight 



Primary Line of Sight and Foveal Chief Ray 



Pupillary Axis and Angle X 



Size of Retinal Image 

 Refraction and Accommodation of the Eye 



Refraction of the Eye 



Accommodation 



Static Refraction of the Eye 



Correction for Ametropia 



Specification of Amount of Accommodation in Play 



Aphakia 

 Ocular Measurements 



Indices of Media 



Purkinje Images 



Optic Axis of the Eye 



Configuration of Front Surface of Cornea 



Measurement of Internal Refracting Surfaces 



X-ray Measurement of Axial Length of the Eye 



X-ray Determination of Location of Second Nodal Point 



Locating Conjugate Focus of Retina 

 Mechanism of Accommodation 



Intraocular Mechanism of Accommodation 



Ciliary Muscle Potential 



Innervation Controlling Accommodation 



Night and Sky Myopia 

 Visual Field 

 Retinal Illuminance 



Light and Illuminance 



Solid Angle 



Luminance 



Retinal Illuminance 



Transmittance of the Eye 



Stiles-Crawford Effect 

 Stray Light in the Eye 

 Blur of Retinal Image 

 Entoptic Phenomena 



THE EYE PLAYS THE TRIPLE ROLE of gathering infor- 

 mation, coding it and relaying it to the brain. In 

 this chapter we are concerned only with the role which 

 the eye plays as an optical device in gathering in- 

 formation. 



In trying to explain this role I have started with a 

 schematic e\e which is free from some of the defects 

 and complications of an actual eye. With this kind of 

 an eye one can explain how an image is formed and 

 what is meant by refracting power, refraction of the 

 eye, size of the retinal image, etc. 



The eye is not like a telescope which can be taken 

 apart to find out how it works. Hence it is necessary 

 to develop approaches which are not needed with an 

 ordinary optical device. For example, the focal length 

 of an eye cannot be measured directly and we have to 

 substitute the concept of refraction to provide an 

 index of an eye's performance as an optical instru- 

 ment. An attempt will be made to explain how this 

 and other measurements are made on a living eye. 

 After explaining these basic concepts and methods of 

 measurement, consideration will be given to the 

 mechanism whereby the eye can change its focus. 



The physiologist is by right inore concerned with 

 the response of the retina to light than the mechanics 

 of applying light to the retina, but there are some 

 special problems that arise in describing the stimulus 

 applied to the retina to which attention must be 

 given. In optics the word illuminance is used to de- 

 scribe the rate at which light is applied to the retina, 

 but the physiologist wants to call this stimulus inten- 



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