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limited by other factors such as scattering, spherical aberration, and the 

 separation of the receptors in the retina. 



In the center of the fovea where the resolution is greatest, the cones 

 are separated by about two microns from center to center. In order to 

 resolve two points of light as separate images, it must be necessary to 

 excite at least two cones while leaving one in between unexcited. Thus, 

 the images on the retina would have to be separated at least four 

 microns from center to center. If it were necessary to have two cones 

 unexcited between the images of the two spots, this number would be 

 increased to six microns. The maximum resolution observed of 

 2 x 1 ~ 4 radians corresponds to a separation between the image centers 

 on the retina of five microns. In other words, the discrete structure of 

 the retinal receptors could be responsible for the lower limit of resolution 

 for persons with the most acute vision. 



The psychophysical processes of recognizing shapes are very complex. 

 However, a minimum requirement for small objects is that the angular 

 separation of their different parts be larger than the limit of resolution. 

 At 25 cm from the eye, an angle of 5 x 10 ~ 4 radians would correspond 

 to about 100 microns. This is about the length of a Paramecium caudatum 

 which should accordingly be recognized as having a rod shape at that 

 distance. In contrast, the smaller species, Paramecium aurelia, would 

 have to be brought closer to the eye before its shape could be recognized 

 by the unaided eye, even under ideal conditions of lighting. 



In a camera, resolution in white light is often limited by chromatic 

 aberration, that is, the different wavelengths focus at different planes. 

 The resolution can be improved to some extent by using a system of 

 positive and negative lenses made of different types of glass. 3 The index 

 of refraction of each will vary in a different fashion with wavelength. 

 By a proper choice, a combination can be made which has a positive 

 focal length that is almost independent of wavelength throughout the 

 visible region. 



Chromatic aberration in the eye is minimized by limiting the wave- 

 lengths of light to which the eye will respond. A bare retina from which 

 the vitreous humor has been removed will respond far into the ultra- 

 violet. However, in the intact eye, the cornea absorbs most energy at 

 wavelengths shorter than 3,000 A. Accordingly, energy at these wave- 

 lengths does not contribute to vision although it can produce corneal 

 damage. 



The crystalline lens has a very sharp cutoff at about 3,800 A. Persons 

 without this lens cannot accommodate to different object distances, lack 

 acuity, but can see objects using ultraviolet radiations only. They have 



3 These are called achromatic lenses. 



