CHAPTER XVIII. 

 ERRORS IN REFRACTION. 



PHYSIOLOGICAL ERRORS. ACCOMMODATION. 

 SPHERICAL ABERRATION. 



In the description of refraction by lenses it has been stated that 

 a spherical lens brings all the rays from one point on the object to 

 a focus at the same point. As a matter of fact this is not strictly 

 speaking the case. The rays which pass through the outer parts 

 of the lenses are more refracted than those nearer the centre; they 

 are therefore brought to a focus a little in front of the central ones, 

 and, passing on, blur the image which the latter form. The differ- 

 ence between the foci becomes greater the nearer the object and the 

 more divergent the rays from it. 



>j Experiment 56. Fill the bottle with water and use it as a lens.* 

 Cover the opening in the lantern by the diaphragm with a 

 2 mm. opening, and set it about a meter away from the bottle. 

 Move the black screen in the pencil of light coming through the 

 lens until you have the light as sharply focussed on it as possible. 

 Note that the region on either side of the focus is dimly lighted. 

 Interrupt the light coming through the outer parts of the lens 

 and note that the focus becomes sharper. Bring the lantern 

 nearer the lens, set the screen at the new focus and interrupt 

 the outer refracted rays as before. The improvement in the 

 sharpness of focus is more marked. 



Cover the round opening of the optical box with the clear 

 glass slide. Set the bottle immediately inside the opening, light 

 the incense in the cork and cover the box. When it has filled 

 with smoke place the lantern about a meter away, so that the 



*The lens in this case is of course a cylindrical one. Since, however, we have 

 to do only with those rays which diverge horizontally, we may use the refraction 

 which the curved surface effects in these to represent refraction by any one plane 

 of a spherical lens. 



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