30 GORHAM, ON THE MAGNIFYING 



falls into the earth's shadow, and becomes eclipsed, or still 

 better in a solar eclipse, when the moon being at her average 

 distance from the earth, the shadow but just reaches the earth's 

 surface. Thus if S (fig. 1) represent the sun, and O the moon, 

 that portion of tlie earth situated at L is in its shadow. 



The shape of a shadow is regulated by the distance between 

 the object and the screen on which the shadow is received. If 

 this be great, the shadow bears no very definite relation in 

 form to that of the object. On the contrary, it is a mere irre- 

 gular darkened space, the boundaries of which are ill defined 

 and the shape distorted. Thus a leaf at the distance of a yard 

 or two from a wall, will, in the sunshine, give a shadow of in- 

 definite outline, having a round instead of an angular edge : a 

 leaf at a greater distance will produce a mere dimness, with an 

 outline scarcely distinguishable. Instances of a like kind are 

 afforded when the sun's rays are obstructed by the topmost 

 branches of a tree, or the summit of a tower, or by the inter- 

 vention of passing clouds, which in their passage through the 

 atmosphere contribute so much to the beauty and variety of 

 the natural landscape, and are amongst those fleeting appear- 

 ances which elude the vigilance of the pencil. 



When the screen is at a great distance from the obstacle, as 

 well as from the source of light, the shadow so far from taking 

 the shape of the obstacle, will resemble that of the luminous 

 body. Thus the shadow of an irregular body placed in the 

 sun's light is circular. 



If on the other hand the object is brought to within a short 

 distance of the screen, its shadow is so clearly defined as to be 

 directly recognized as an exact facsimile, in shape, of the 

 body itself. A leaf nearly close to the wall casts a shadow 

 of a leaf. 



"These observations regarding shadows are applicable to 

 the illuminated space formed on a screen by making the sun's 

 light pass through an aperture." This will be obvious, on 

 reflecting, that if a shadow or darkened space be well defined, 

 the adjacent, illuminated space must be equally so, and vice 

 versa. For these are contrasted conditions, each of which 

 causes the other to become visible. Neither light alone, nor 

 darkness alone, but only contiguity of both will enable us to 

 appreciate form. Hence light and shade are not only pleasant 

 to the eye, but both are absolutely necessary for the distin- 

 guishing of one object from another. For this reason, pro- 

 bably, the visual organ is ever intuitively on the search for 

 contrasts either of light, shade, or colour. 



" When the screen is near the aperture, the illuminated 

 portion is similar to the opening ; but when the screen is suffi- 



