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the Cause of coloured concentric Rings. 79 



together, will give four shadows, one from each upper sur- 

 face and one from the bottom of the lowest of them. 



In all these cases a metalline mirror may be laid under the 

 same arrangement without adding to the number of sha- 

 dows, its effect being only to render them more intense and 

 distinct. 



The shadows may be distinguished by the following me- 

 thod : When the point of the penknife is made to touch the 

 surface of the uppermost glass or lens, it will touch the 

 point of its own shadow, which may thus at any time be 

 easily ascertained ; and this in all cases I call the first sha- 

 dow ; that which is next to it, the second ; after which 

 follows the third, and so on. 



In receding from the point, the shadows will mix to- 

 gether, and thus become more intense ; but which, or how 

 many of them are united together, may always be known 

 by the points of the shadows. 



When a shadow is to be thrown upon any required place, 

 hold the penknife nearly half an inch above the glasses, and 

 advance its edge foremost gradually towards the incident 

 light. The front should be held a little downwards to keep 

 the light from the underside of the penknife, and the sha* 

 dows to be used should be obtained from a narrow part of it. 



With this preparatory information it will be easy to point 

 out the use that is to be made of the shadows when they 

 are wanted. 



IV. Of two Sets of Rings, 



I shall now proceed to describe a somewhat more com- 

 plicated way of observation, by which two complete sets of 

 concentric rings may be seen at once. The new or addi* 

 tional set will furnish us with an opportunity of examining 

 rings in situations where they have never been seen before, 

 which will be of eminent service for investigating the cause 

 of their origin, and with the assistance of the shadows to 

 be formed, as has been explained, we shall not find it diffi- 

 cult to see them in these situations. 



First Method, Upon a well polished piece of good look- 

 ing-glass lay down a double convex lens of about 20 inches 

 focus. When the eye-glass has been adjusted as usual for 



seemcf 



