668 THE POPULAR SCIENCE MONTHLY. 



the drops the broader are the zones of the supernumerary bows, and 

 Young proved by calculation that when the drops have a diameter 

 of ToTs or :roV{r o^ ^^ mc\ the bands overlap each other, and pro- 

 duce white light by their mixture. Unlike the geometric bow, the 

 radius of the white bow varies within certain limits, which M. Bravais 

 shows to be 33° 30' and 41° 46' respectively. In the latter case the 

 white bow is the ordinary bow deprived of its color by the smallness 

 of the drops. In all the other cases it is produced by the action of the 

 supernumeraries. 



The physical investigator desires not only to observe natural phe- 

 nomena but to recreate them — to bring them, that is, under the domin- 

 ion of experiment. From observation we learn what Nature is willing 

 to reveal. In experimenting we place her in the witness-box, cross-ex- 

 amine her, and extract from her knowledge in excess of that which 

 would, or could, be spontaneously given. Accordingly, on my return 

 from Switzerland last October, I sought to reproduce in the laboratory 

 the effects observed among the mountains. My first object, therefore, 

 was to obtain artificially a mixture of fog and drizzle like that ob- 

 served from the door of our cottage. A strong cylindrical copper 

 boiler, sixteen inches high and twelve 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 opened, 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, the 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 ob- 

 server stood, with his back to the light, between them. It is not, how- 

 ever, necessary to dwell upon these first experiments, surpassed as they 

 were by the arrangements subsequently adopted. My mode of pro- 

 ceeding 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 center 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 



