RAINBOW. 675 KAIN GAUGE. other colors ranged between. Of course, a particular drop is only in the right ])osition to contribute to this ring of light for a small frac- tion of a second, but others take its place. Jloreover, the apparent diameter of the sun causes a widening of the line of light to a band, and these bands of different colors overlap and blend. The bow formed as above described is called the 'primary bow' and is )iiuch brighter than the "secondarv bow,' which is formed as Fio. 3. follows: By a line of reasoning entirely an- alogous to that given above, it may be shown that light falling upon the opposite half of the drop, as shown in Fig. 3, may undergo two reflections in the drop and emerge in the direc- tion DT. Again, when the path is symmetrical in the drop, the angle between SB and DT is now smaller than for any other case, but larger for violet than for red, and in all cases larger than the angle between SB and CT in the pri- mary bow. Applying a similar consideration to Fig. 4, and considering it revolved upon the line TS' as an axis, it will be seen that colored bands of light will be seen by the eye at T of an angular diameter greater than any in the pri- marj' bow, and with the violet having the largest and the red the smallest diameter. The angular radius of the red is about 50° 24' and of the violet 53° 22'. The values of the diameters were calculated by Airy and experimentally veri- fied by ililler. When the conditions are very favorable other fainter bows may be seen inside the primary and outside the secondary bow. These correspond to a still more comple.x com- bination of reflections and refractions of the rays in the drops. A lunar rainbow differs from a solar bow only in the intensity of the light and consequent paleness of the colors. RAINBOW FALLS. A cataract in the State of Washington. See ('IIELAX, L.KE. RAINBOW TROUT. The trout of the coast ranges of the Pacific Coast, from central Cali- fornia northward to Puget Sound, which takes its name (.S'u/mo irridens) from the large, bril- liant spots scattered over its bluish-silver3' body. See Trout : and Plate of Trout .vnd Grayung. RAIN CROW. See A.i ; and Plate of CucKOO. RAIN GAUGE. An apparatus to catch the rain and measure its equivalent depth on thef ground at any place. The simplest rain gauge consists of a vertical cylinder, into which one may dip a graduated stick and read off the depth of the collected water. Such an instru- ment is furnished by the United States Weather Bureau to its voluntary observers, and is illus- trated in the accompanying figure. Other gauges consist essentially of funnels to catch the water, glass bottles to hold it, and slender grad- uated glass tubes by which to make minute Front View Vertical Section i FV dj '^ e c D > u™ Receiver Hori7ontaSecnoniF, .MPLE KA1. GArGE, i:.8. WEATHER BDKEAC PATTEEN. measurements. Self-registering gauges are now made so economically that they are always to be recommended: those of the simplest pattern em- ploy a tipping bucket, divided into two equal eonipartments. and tipping to the right and left alternately, according as the right or left com- partment is filled. Each tip corresponds to a definite depth, such as a hundredth of an inch. Another favorite form consists essentially of a weighing machine, and every ounce of rain, or the equivalent depth, is properly recorded on a. sheet of paper moved by clockwork. Both forms, as perfected by Proftesor C. F. Marvin, are used by the United States Weather Bureau. It has long been known (since Heberden, 1776) that perfectly reliable gauges collect less rain the greater their height above ground, and it is now satisfactorily demonstrated that this is entirely due to the fact that the gauge causes eddies of wind around and even within its
