28 



GORHAM, ON THE MAGNIFYING 



for instance, is allowed to intervene ; it is evident that a 

 shadow of the cross in the latter case, and an illuminated space 

 equivalent to the shadow in size and shape, in the former, is 

 portrayed on the retina of the eye. 



The same kind of phenomena result even if no artificial 

 body be interposed between the eye and the source of light, 

 the pupillary aperture in this case constituting the transparent 

 space, and the i7'is the blackened margin which gives it outline, 

 so that those rays which are not intercepted by this curtain, 

 pass onwards and ultimately form a picture of the pupil itself 

 at the bottom of the eye. 



When an opaque object is held either in a beam of light 

 (bundle of parallel rays), or a pencil of light (rays proceeding 

 from or towards some point), it intercepts a portion of the 

 rays, and the space behind the object is in darkness. This 

 dark space is called the shadow of the object. Thus in figure 

 1, if the luminous body L emits a pencil of light which is 

 stopped in its passage towards the screen by a round piece of 

 blackened pasteboard, O, the dark space between this and the 

 screen, W, is the shadow. 



Fig. 1. 



A shadow may be received on a screen held near the object, 

 when its outline will be similar to that of the body by which 

 it is cast. Thus the shadow of the circle O (fig. 1,) is pro- 

 jected as a circle at S, on the white screen W. 



The breadth of a shadow depends on the direction and dis- 

 posal of the rays of light when they are stopped by the oppo- 

 sing body. These may be 2)arallel, divergent, or convergent. In 

 the remarks which immediately follow, I shall merely embody 

 so much under each of these heads as relates to the subject of 

 our present disquisition. 



With respect to jmrallel rays it is to be observed, that the 

 farther the luminous body is from an object, the less divergent 



