256 ADAPTATIONS TO SPACE AND MOTION 



means of forming an image upon the retina, instead of the pinhole as 

 Nautilus (Fig. Id, p. 3) chose to do. As Figure 89 (p. 224) shows, the 

 pinhole is a much simpler gadget than the lens, and the image it forms 

 is quite sharp when caught on a screen at any reasonable distance. But 

 it has one very great disadvantage: the amount of light, emanating from 

 an object-point, which can form a corresponding image-point, is just the 

 slender pencil of rays which get through the pinhole. Apparently, this 

 pencil should be a single ray if the image is to be maximally sharp, and 

 the size of the pinhole would then be a quite impractical, mathematical 

 point. 



Actually however, as the pinhole is made smaller and smaller, the 

 image at first sharpens but finally becomes more and more blurred 

 through the introduction of diffraction. The optimal diameter of a pin- 

 hole aperture is equal to twice the square root of the product of the 

 screen-distance and the wavelength of the light. An ideal pinhole located, 

 say, at the position of the inner surface of the cornea in a lensless human 

 eye, would need to be 0.23 millimeters in diameter; and a point four 

 inches from the eye would then be imaged on the retina as a one- 

 millimeter circle. 



A lens gathers in a cone of light-rays from each point of the object, 

 and converges all of this light again to form a point in the image, which 

 is hence far brighter than the one formed by a stenopaic aperture. Other 

 things being equal, the broader the lens, the brighter the image. Where 

 a pinhole is employed to eliminate the need for accommodation at cer- 

 tain times or all the time, the retina must be very sensitive even though 

 the stenopaic aperture is used in bright light. A reduction of the need 

 for accommodation — what a photographer would call a deepening of the 

 focus of the eye — is an incidental gain of any slit-pupilled animal; for a 

 slit, like a round pinhole, is to be considered a stenopaic aperture, al- 

 though an astigmatic one. And, slit-pupilled vertebrates always have the 

 necessary extra sensitivity in their retinae to make vision remain bright 

 enough when the slit is closed down. That is why they have the slit. 



Animals whose pupils are specially designed to provide stenopaic aper- 

 tures include Scylliorhinus (Fig. 91, p. 225), Ra]a (Fig. 65, p. 158), 

 geckoes (Fig. 88, p. 223; and see p. 224), some ungulates (especially 

 camelids — Fig. 86c, p. 219), the domestic cat, and Paradoxurus. Still 

 others have the benefit of a pinhole in bright light, though their phe- 

 nomenally contractile circular pupils are no doubt intended primarily to 

 shield an extra-sensitive retina — a job which ordinarily calls for a slit 



