464 Professor Tyndall [Jan. 18, 



therefore, was to obtain artificially a mixture of fog and drizzle like 

 that observed from the door of our cottage. A strong cylindrical 

 copper boiler, 16 inches high, and 12 inches in diameter, was nearly 

 filled with water, and heated by gas flames until steam of twenty 

 pounds pressure was produced. A valve at the top of the boiler was 

 then ojDened, when the steam issued violently into the atmosphere, 

 carrying droplets of water mechanically along with it, and condensing 

 above to droplets of a similar kind. A fair imitation of the Alpine 

 atmosphere was thus produced. After a few tentative experiments, 

 the luminous circle was brought into view, and having once got hold 

 of it, tlie next step was to enhance its intensity. Oil lamps, the lime- 

 light, and the naked electric light were tried in succession, the 

 source of rays being placed in one room, the boiler in another, while 

 the observer stood, with his back to the light, between them. It is not, 

 however, necessary to dwell upon these first experiments, surpassed as 

 they were by the arrangements subsequently adopted. My mode of 

 proceeding was this. The electric light being placed in a camera 

 with a condensing lens in front, the position of the lens was so fixed 

 as to produce a beam sufficiently broad to clasp the whole of my head, 

 and leave an aureole of light around it. It being desirable to lessen 

 as much as possible the foreign light entering the eye, the beam was 

 received upon a distant black surface, and it was easy to move the 

 head until its shadow occupied the centre of the illuminated area. To 

 secure the best effect it was found necessary to stand close to the 

 boiler, so as to be immersed in the fog and drizzle. The fog, however, 

 was soon discovered to be a mere nuisance. Instead of enhancing, it 

 blurred the effect, and I therefore sought to abolish it. Allowing the 

 steam to issue for a few seconds from the boiler, on closing the valve, 

 the cloud rapidly melted away, leaving behind it a host of minute 

 liquid spherules floating in the beam. A beautiful circular rainbow 

 was instantly swept through the air in front of the observer. The 

 primary bow was duly attended by its secondary, with the colours, 

 as usual, reversed. The opening of the valve for a single second 

 causes the bows to flash forth. Thus, twenty times in succession, jDuffs 

 can be allowed to issue from the boiler, every puff being followed by 

 this beautiful meteor. The bows produced by single puffs are 

 evanescent, because the little globules rapidly disappear. Greater 

 permanence is secured when the valve is left ojDen for an interval 

 sufficient to discharge a copious amount of drizzle into the air.* 



* It is perhaps worth noting here, that when the camera and lens are used, 

 tlie beam which sends its " effective rays " to the eye may not be more than a foot 

 in widtl), while the circular bow engendered by these rays may be, to all appear- 

 ance, fifteen or twenty feet in diameter. In such a beam, indeed, the drops which 

 produce the bow must be very near the eye, for rays from the more distant drops 

 would not attain the required angle. The apparent distance of the circular bow 

 is often great in comparison with that of the originating drops. Both distance 

 and diameter may be made to undergo variations. In the rainbow we do not see 

 a localised object, but rcctive a luminous impression, which is often transferred 

 to a portion of the held of view far removed from the bow's origin. 



