670 THE POPULAR SCIENCE MONTHLY. 



tints, while the secondary, though less vivid, shows its colors in sur- 

 prising strength and purity. 



But the primary bow is accompanied by appearances calculated to 

 attract and rivet attention almost more than the bow itself. I have 

 already mentioned the existence of effective rays over and above those 

 which go to form the geometric law. They fall within the primary, 

 and, to use the words of Thomas Young, " would exhibit a continued 

 diffusion of fainter light, 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 literally his words are ful- 

 filled, and how beautifully his theory is illustrated, by these artificial 

 circular rainbows. For here the space within the primaries is swept 

 by concentric supernumerary bands, colored 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 splendid illustrations of the principle of interference, are 

 separated from each other by zones of darkness, where the light- 

 waves, on being added together, destroy each other. I have counted 

 as many as eight of these beautiful bands, concentric with the true pri- 

 mary. The supernumeraries are formed next to the most refrangible 

 color of the bow, and therefore occur within the primary circle. But, 

 in the secondary bow, the violet, or most refrangible color, 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 employed 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 laterally, and breaks up into ex- 

 ceedingly 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 density were required. In regard to primaries, 

 secondaries, and supernumeraries, extremely brilliant effects have been 

 obtained with this form of spray-producer. The quantity of water 

 called upon being much less than that required by the rose, the fillet- 

 and-disk instrument produces less flooding of the locality where the 

 experiments are made. In this latter respect, the steam-spray is par- 

 ticularly handy. A puff of two seconds' duration suffices to bring 

 out the bows, the subsequent shower being so light as to render the 

 use of water-proof clothing unnecessary. In other cases, the incon- 

 venience of flooding may be avoided to a great extent by turning on 

 the spray for a short time only, and then cutting off the supply of 

 water. The vision of the bow being, however, proportionate to the 



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



