HERSCHEL ON COLOURED RINGS. \OJ 



frill be darker than the first, but the third will be faint. 

 When a piece of looking-, glass is substituted for the lowest 

 slip* the third shadow will be the strongest. 



Three slips of glass in contact, or two slips with a lens Four from 

 upon them, or also a looking glass, a slip and a lens put toge- three glasses. 

 (her, will give four shadows, one from each upper surface and 

 one from the bottom of the lowest of them. 



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

 the same arrangement without adding to the number of sha- renders them 

 dows, itseifect being only to render them more iutense and dl 

 distinct. 



The shadows may be distinguished by the following me- Method of dis- 

 thod. When the point of the penknife is made to touch the tinguishing 

 surface of the uppermost glass or lens, it will touch the point the shadow >» 

 of its own shadow, which may thus at aey time be easily as- 

 certained: and this in all cases I call the first shadow ; that 

 which is next to it, the second ; after which follows the third, 

 and >o on. 



In receding from the point, the shadows will mix together, Mixture of ih« 

 and thus become more intense; but which, or how many of shadows. 

 them are united togther, may always be known by the points 

 of the shadows. 



When a shadow is to be thrown upon any required place, p recaut i ons 

 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 compli- Two sets of 

 cated way of observation, by which two complete sets of r,n S s « 

 concentric rings may be seen at once. The new or additi- 

 onal set will furnish us with an opportunity of examining 

 lings in situations where they have never been seen before, 

 which will be of eminent service for investigating the cause 

 of their origin, land with the assistance of the shadows to be 



formed, 



