10 



VISUAL ANGLE 



[Ch.I 



constantly appearing larger. The closer one gets the smaller is the 

 visible field, but the larger will the parts seem to be. If the hole is 

 quite small, one can get the object within 4 or 5 cm. of the eye and still 

 see the image clearly, and see details which could not be seen at a 

 greater distance. 



As shown in the figures of the visual angle (fig. 76), the closer the eye 

 gets to the object the greater will be the visual angle, hence details 



are shown which did not appear at a 

 greater distance. One of the best meth- 

 ods of trying this experiment is to use for 

 object a small mark made with ink or a 

 glass pencil on a window or on a milky 

 or transparent lamp shade. Then there 

 win be plenty of Hght. The physiologi- 

 cal explanation of the power to see 

 clearly through the pinhole at a distance 

 of 5 cm., when, if the eye looks directly 

 at the object, it should be about 25 cm. 

 from the eye, is, that with the pin-hole 

 the beam is so narrow that it affects so 

 narrow a circle on the retina that the 

 appearance is like a good focus. If one 

 takes away the card, the beam gets very 

 wide and the eye has only a blurred 

 impression, the diffusion circles are so 

 large. 



§ 7a. In case one loses his spectacles or has the accommodation paralyzed 

 by atropin for testing the eyes, it is possible to read fairly well with the per- 

 forated card if the print is in a brUliant light. The field which" can be seen at 

 one time is very small, so one must move the print or the head almost constantly. 



Fig. 7. Pin-hole Card for 

 Viewing Near Objects. 



Lenses 



The usual and most effective means for increasing the visual angle 

 when examining small objects is by the use of lenses, singly or in com- 

 bination. 



§ 8. Lens. — A lens means a mass of transparent glass or other 

 substance with one plane and one curved, or with two curved sur- 



