i66 Professor Tyndall [Jan 18, 



I Lave 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 therefore occur within the 

 primary circle. But in the secondary bow, the violet, or most re- 

 frangible 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 emj)loyed 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 breadth and density were required. In regard to 

 primaries, secondaries, and supernumeraries, extremely brilliant eifects 

 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 

 particularly handy. A puff" of two seconds' duration suflSces to bring 

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

 use of waterproof 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 

 duration of the shower, will, when the shower is brief, be evanescent. 

 Hence, when quiet and continued contemplation of all the phenomena 

 is desired, the observer must make up his mind to brave the rain.* 



In one important particular the spray-producer last described 

 commends itself to our attention. With it we can operate on sub- 

 stances more costly tban water, and obtain rainbows from liquids of 

 the most various refractive indices. To extend the field of experiment 

 in this direction, the following arrangement has been devised: A 

 strong cylindrical iron bottle, wholly or partly filled with the liquid 

 to be experimented on, is tightly closed by a brass cap. Through 

 the cap passes a metal tube, soldered air-tight where it crosses the 

 cap, and ending near the bottom of the iron bottle. To the free end 

 of this tube is attached the sjiray-producer. A second tube passes 

 also through the cap, but ends above the surface of the liquid. This 

 second tube, which is long and flexible, is connected with a larger 

 iron bottle, containing compressed air. Hoisting the small bottle to 

 a convenient height, the tap of the larger bottle is carefully opened, 

 the air passes through the flexible tube to the smaller bottle, exerts 



* Tlie rays which form the artificial bow emerge, as might be expected, 

 polariacd from the drops. 



