TO THE MICROSCOPE. 



271 



Colours of Selenite for different thicknesses.— We hence see 

 that the colour of the selenite depends on its thickness, and the 

 above examples show how it would be possible to calculate out 

 from first principles what would be the colour for a7iy thickness. 

 The sequence of colours for different thicknesses, beginning with 

 the thinnest, is given in the following table : — 



Colours of Selenite or Newton's Rings. 



