l68 THE SENSES OF ANIMALS 



for the diameter of the opening relative to the overall size 

 is too great to act as a pin-hole lens, but the more complex 

 open eye of the nautilus is one of the very few examples of 

 an eye which probably does form an image with a pin-hole 

 and not with a refracting lens. 



A pin-hole can be a very useful improvised seeing-aid to 

 a long-sighted person who has mislaid his spectacles ; if he 

 makes a pin-hole in a thick piece of paper or thin card and 

 holds it close to his eye he can read the smallest print through 

 it perfectly well, provided the light is bright. Mr. Pepys, 

 who had to give up keeping his diary because of failing sight, 

 used what he called his "tubes" for reading and writing — 

 no doubt these were tubes with pin-hole lenses at the eye- 

 ends, for the use of a tube with a pin-hole increases the 

 apparent sharpness of the image by excluding stray light 

 from the sides. 



The disadvantage of the pin-hole lens is that it transmits 

 only a small quantity of light ; if, as we believe, the evolution 

 of eyes started from light-gathering organs that had at first 

 no visual function it is not surprising that the refracting lens 

 has almost universally been adopted for image forming racher 

 than the diffracting pin-hole. 



