1884.] on Bainhows. 465 



Many other apj^liances for producing a fine rain have boon tried, 

 but a reference to two of them will suffice. The rose of a watering- 

 pot naturally suggests a means of producing a shower ; and on the 

 principle of the rose I had some spray-producers constructed. In 

 each case the outer surface was convex, the thin convex metal plate 

 being pierced by orifices too small to be seen by the naked eye. 

 iSmall as they are, fillets of very sensible magnitude issue from the 

 orifices, but at some distance below the spray-producer the fillets 

 shake themselves asunder and form a fine rain. The small orifices 

 are very liable to get clogged by the particles suspended in London 

 water. In experiments with the rose, filtered water was therefore 

 resorted to. A large vessel was mounted on the roof of the Royal 

 Institution, from the bottom of which descended vertically a piece of 

 compo-tubing, an inch in diameter and about twenty feet long. By 

 means of proper screw fittings, a single rose, or, when it is desired to 

 increase the magnitude or density of the shower, a group of two, 

 three, or four roses, is attached to the end of the compo-tube. From 

 these, on the turning on of a cock, the rain descends. The circular 

 bows produced by such rain are far richer in colour than those 

 produced by the smaller globules of the condensed steam. To see the 

 effect in all its beauty and completeness, it is necessary to stand well 

 within the shower, not outside of it. A waterproof coat and cap are 

 therefore needed, to which a pair of goloshes may be added with 

 advantage. A person standing outside the beam may see bits of both 

 primary and secondary in the places fixed by their respective angles ; 

 but the colours are washy and unimpressive, while within the shower, 

 with the shadow of the head occupying its proper position on the 

 screen, the brilliancy of the effect is extraordinary. The primary 

 clothes itself in the richest tints, while the secondary, though less 

 vivid, shows its colours in surprising 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 bow. 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 fulfilled 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, coloured like the rain- 

 bow, 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 li^^ht waves, on being added together, destroy each other. 



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



