LIGHT. 129 



four or five inches focus, project the piece in the same 

 manner precisely as you would with the solar micro- 

 scope. The prism must be held in the focus of the 

 objective always. 



5. Geometrical designs in mica. 



Choose a thin plate of mica 

 that is clear, and three inches 

 or more square. Hold it in the 

 polarized light and see if it pre- 

 sents lively colors ; it will if it 

 is thin enough. It ought not to 

 exceed the fiftieth of an inch in 

 thickness for best effect. When 

 the tint appears uniform over Fiff * 96t 



its whole surface, as it will if its thickness is uniform, 

 it may have drawn upon it with a lead pencil such a 

 figure as the accompanying one, and then trim to 

 the edge of the circle with scissors. Afterwards, with 

 a sharp pen-knife cut about one fourth of the way 

 through the mica on all the lines ; then with a needle 

 point start to split the point on the edge. When a 

 thin leaf has been raised a little between two points, 

 carefully move the needle round the edge so as to 

 separate the same thickness all around the circumfer- 

 ence. Do not disturb the points of the star more than 

 at the extreme point, just enough to keep the needle 

 in the same layer. If the knife has cut through this 

 layer that has been raised at the edge, the parts o, 0, o, 

 can be removed, leaving the six-pointed star a little 

 raised above the surface o, o, o. 



Examine this now with the polarized light, and the 



star will appear to be of one color and the cut-away 



parts of another. If the interior part c be removed it 



is very probable that that part will exhibit still another 



9 



