AND ITS CONGENERS. 217 



but for the general law of interference which divides 

 the light into concentric rings." One could almost 

 wish for the opportunity of showing Young how lit- 

 erally his words are fulfilled, and how beautifully his 

 theory is illustrated, by these artificial circular rain- 

 bows. For here the space within the primaries is 

 swept by concentric supernumerary bands, coloured 

 like the rainbow, and growing gradually narrower as 

 they retreat from the primary. These spurious bows, 

 as they are sometimes called,* which constitute one of 

 the most striking illustrations of the principle of in- 

 terference, are separated from each other by zones of 

 darkness, where the light-waves on being added to- 

 gether destroy each other. I have counted as many 

 as eight of these beautiful bands, concentric with the 

 true primary. The supernumeraries are formed next 

 to the most refrangible colour of the bow, and there- 

 fore occur within the primary circle. But in the second- 

 ary bow, the violet, or most refrangible colour, is on 

 the outside; and, following the violet of the secondary, 

 I have sometimes counted as many as five spurious 

 bows. Some notion may be formed of the intensity 

 of the primary, when the secondary is able to produce 

 effects of this description. 



An extremely handy spray-producer is that em- 

 ployed to moisten the air in the Houses of Parliament. 

 A fillet of water, issuing under strong pressure from 

 a small orifice, impinges on a little disk placed at a 

 distance of about one-twentieth of an inch from the 

 orifice. On striking the disk, the water spreads lateral- 

 ly, and breaks up into an exceedingly fine spray. Here 

 also I have used the spray-producer both singly and 

 in groups, the latter arrangement being resorted to 

 when showers -of special breadth and density were re- 



* A term, I confess, not to my liking. 



