AND THE THKOHY OF LIGHT. 89 



the atmosphere. Others may be prevented from forming 

 the same idea in consequence of the too exact geometrical 

 forms which the different strata assume in the experiment. 



Supposing the beam of light from the card to generate 

 a prism, or the section of a lens, one on each side of the 

 axis, we may then imagine the parallelograms on the left 

 side of the axis to be figures seen through the left side 

 of the axis of the prism, and those on the right to be 

 figures seeu through the right side of the axis; that 

 is through the two angles on the right and left of the 

 axis of the prism. They obey the same law as regards 

 colour. 



Is not this a most important step towards an explanation 

 of prismatic refraction ? 



138. These experiments maybe infinitely varied. There 

 is, however, another which I cannot avoid mentioning. 



On a piece of pasteboard draw two circles. {See plate 

 V. fig. 9.) The outer circle may be three and a half 

 inches in diameter, the inner a quarter of an inch or so 

 less, which will give a ring about an eighth of an inch 

 broad. Draw two lines, one on each side of the diameter, 

 so as to inclose a space of about ^e of ^^ inch, broad. Cut 

 the circle, a a a, out of the board, and the segments b c 

 out of the circle, leaving the space c? c? as a diameter. 

 Fix the figure at o on the diameter, either perpendicularly 

 or at an angle, and examine the different eflects. 



When made to revolve in sunshine the colours at times 

 are most exquisite. The whole figure is as elegant as 

 imagination can conceive. It is not so opaque as mother 

 of pearl, or as soap bubbles, but it partakes of all their 

 evanescent colours. Attention is particularly directed in 

 this experiment to the shadows which are seen on the 

 generated figure, cast by the edges of the figure itself. 

 They are very remarkable, and illustrate, I suspect, many 

 of the phenomena which are to be seen in crystals and 



SER. III. VOL. I. N 



