R66 



RAINBOW 



RAIN-GAUGE 



other occur. Two bows are frequently seen, each 

 exhibiting the full spectrum of colours from 

 red to violet ; hut in the inner or primary 

 bow the red is the outer edge and violet the 

 inner, while in the outer or secondary bow the 

 order is reversed, the red being inside and the 

 violet on the exterior. The colours are always 

 arranged in a definite order, that of the solar 

 spectrum viz. red, orange, yellow, green, blue, 

 indigo, and violet, but shade imperceptibly into 

 each other. The cause of this breaking up of the 

 sunlight into its constituent colours is explained 

 in most physical and meteorological text-books (see 

 Light, by Professor Tait, chap, x., or Meteorology, 

 by E. Loomis, par. 416), but may be briefly sum- 

 marised as follows* : 



For the primary bow (fig. 1), let PQR represent 

 the section of a raindrop, and SP a ray of light 

 falling on it. The ray enters the drop at P, meets 



Fig. L 



the surface again at R, is reflected to Q, where it 

 leaves the drop in the direction QE. The ray is 

 refracted or bent on entering the drop at P and 

 again on emerging at Q the amount of this refrac- 

 tion depending on the acuteness of the angle at 

 which the ray meets the surface. Now it may be 

 shown that there is a particular point P, such that 

 any ray from S striking the surface below P 

 emerges again above Q, and any ray alxm* P also 

 emerges above Q the former owing to the more 

 acute angle of the reflection, and the latter to the 

 greater refraction on entering and leaving the 

 drop. The course of two sucn rays U shown by 

 the dotted lines in fig. I. Q is thus a turning- 

 point in the emerging rays, ami near it a very 

 large number of rays pox* out, and an observer at 

 E sees a bright image of S in the direction EQ. 

 This statement applies to any one colour of sun- 

 light ; but, as the refrangihility increases from red 

 to violet, the latter is bent more at P and Q, and 

 the line EQ lies at a flatter angle. The oliserver, 

 therefore, sees the violet rays reflected on drops at 



Fig. 2. 



a less altitude than those that reflect the red, the 

 other colours beinj; intermediate. The raindrop 

 being spherical, tin- reflection takes place in all 



directions, the fixed condition iK-ing the radius of 

 the bow, that i- the angle between the line from 

 the oWrver to the bow and that passing from the 

 sun to the observer.or, in other words, the observer's 

 shadow. For red light this angle is 42 39', and for 

 violet 40 13'. If the sun were a luminous point 

 each colour would lie sharply defined, hut as the 

 disc of the sun subtends an angle of about 30* 

 each colour is broadened to this amount, and they 

 overlap. 



Exactly similar .reasoning explains the secondary 

 bow (fig. 2). The light Unit forum it has been 

 twice reflected, at K and at R', the point Q lies 

 above P, and rays entering either above or below 

 P all emerge below Q. A glance at the diagram 

 will show that the greater pending of the more 

 refrangible rays makes the line EQ more nearly 

 vertical, and therefore the violet rays form tin- 

 outer edge and the red the inner of the secondary 

 IKIW. The radius of the red is 50 5', and of the 

 violet 54 0'. The space between the bows gets no 

 reflected light, but that inside the primary ami 

 outside the secondary is faintly illuminated by 

 rays such as are indicated by the dotted lines in 

 fig. 1 and their equivalents in fig. 2, which are 

 not shown. These rays ' interfere (see INTERFER- 

 ENCE) with each other, and cause alternations of 

 colour which appear as spurious bows inside the 

 primary and outside the secondary. They can 

 only be seen with strong sunlight and small drops 

 of rain. 



The radius of the primary bow being roughly 40, 

 it is evident that it cannot be seen when the sun is 

 at a greater elevation than this, as the highest 

 part of the bow would lie below the horizon. 

 Hence in the latitude of Edinburgh rainbows 

 cannot be seen for several hours about noon at the 

 time of the summer solstice. If the drops of water 

 be very small the interference of the rays causes 

 such a complete overlapping of the colours that 

 the bow appears white : this is the case generally 

 with a fog-bow. 



Intersecting rainbows have frequently been seen. 

 When the sun is reflected from a surface of still 

 water a bow is formed by the reflected image as 

 well as by the sun itself. Lunar rainbows often 

 occur, but the feebleness of the moon's light usually 

 prevents any colours being observed. There are 

 many popular weather prognostications connected 

 with rainbows, all dependent on the fact that they 

 imply local passing showers. ' A rainbow in the 

 morning is tin- shepherd's warning : a rainbow at 

 nijiht is the shepherd's delight, 'is easily understood 

 when we remember that the rainbow is formed 

 omMwdte the sun, and that weather-changes in 

 the British Islands generally pass from west to 

 Met, 



Rnincy's Corpuscles. See GREOARINIDA. 



Rnill-KiltlKC. Rain-gauges measure the quan- 

 tity of rain which falls, and are of various con 

 structions. One of the best is that known as 

 Symons' rain-gauge, consisting of a funnel-shaped 

 ri-rriving vessel, and a glass measure of much 

 smaller diameter so as to allow as nice graduation 

 as may be desired. In the fig. a is the funnel- 

 nha|>ed receiving vessel (shown in section in the 

 tig. ), 5 inches in diameter, with an upright, sharp- 

 edged and strong rim, in order that it may return 

 its circularity ; b, tin- vessel which receives the 

 rain collected ; and c, the graduated glass measure 

 which measures the amount in tenths and linn 

 dredths of an inch. If desired it may IH> graduated 

 to still greater nicety. Another excellent gauge 

 ii one whirh is a modification of (Jlaishor's gauge, 

 brought into use by tin- Meteorological * Illice. '1 he 

 diameter of this gauge is 8 inches, and there is 

 added to it a vertical cylinder on the top of the 



