128 



Diffraction Experiments. 



second grating should then be removed and the effect of the 

 slit-disc observed, first with one and then with two gratings. 

 By holding the tube in the middle, either the eye-piece or the 

 disc holder may be rotated separately as desired.. 



If a disc pierced with symmetrically-arranged holes in a 

 circle about the size of a shilling, is employed, a very striking 

 effect of perspective is noticed. The holes are really in one 

 plane at right angles to the eye of the spectator ; but when one 

 grating is employed, each hole is -seen in the centre of a spindle 

 of many coloured light, and all the spindles seem arranged on a 

 horizontal cylinder, so that when one of them looks exactly before 

 the eye, and near it, another seems precisely behind the first 

 and farther off. As the disc holder is rotated, the cylinder of 

 spindles revolves, each spindle alternately approaching and 

 receding from the eye. When one spindle looks exactly behind 

 another, it is impossible to avoid the illusion that its light is 



A. Holder for ruled glass, etc. B. Screen. C. Box holding apparatus. 



literally seen through the light of the other. A singularly beauti- 

 ful addition to the effect is obtained by the use of the second 

 ' c grating" at right angles to the first. The rotation of the 

 disc holder then gives the spectacle of two cylinders of spindles 

 rotating at right angles to each other, and crossing their lines 

 of light in a magical way. 



Instead of the disc of pin-holes in a circle, another disc 

 with a large central hole and smaller ones removed from it may 

 be used. This planet and satellite disc is very fine in its 

 effects, especially when the slender light spindles afforded by 

 the small holes appear to be behind and seen through the big 

 spindle which the large hole gives rise to. Holes arranged in 

 a triangle are likewise interesting ; but any one who possesses 

 the " Diffraction Kaleidoscope" can vary the patterns of the 

 discs with no more trouble than piercing some discs of 

 blackened card. 



When the disc with pin-holes in a circle is used with two 



