THE RAI-VDOW.] 



METEOROLOGY. 



1149 



of two classes, solar and lunar ; the latter, however, are 

 rare ; and even when they do occur, the bow is seldom 

 coloured. 



Fig. ST. 



The cliicf conditions under which a solar rainbow may 

 occur, are the following : The sun must not have less 

 than 42 of angular elevation ; the back of the spectator 

 must he towards the sun, and rain must be falling from 

 a highly illuminated cloud. The rainbow is usually 

 dc.in.lc, and the theory of its formation may be thus ex- 

 plained. Let it be assumed that a straight line passes 

 from the eye of the observer through the sun ; then this 

 line will constitute the axis of a cone, the base of which 

 will be the rainbow, and its vertex the eye of the 

 observer. If the bow be at the horizon, and the place of 

 observation be a level plain, then the rainbow will appear 

 as a perfect circle constituting the base of the cone. 

 This complete circular appearance is, however, rare, the 

 rainbow being far more generally, as its name implies, a 

 mere arc of coloured light. The explanation is now 

 evident "f the fact that the rainbow cannot appear when 

 the snn has attained an elevation greater than 45. The 

 rainbow, though still depicted, is depicted below the 

 horizon, and is, therefore, invisible. It follows, then, 

 that the size of the visible rainbow is inversely as the 

 elevation of the sun above the horizon. 



The following diagram (Fig. 58) illustrates the forma- 

 tion of the rainbow. The straight line A P is supposed 

 to be drawn between the sun and the eye of the observer, 

 passing through the latter. Through this lino a vertical 

 plane is supposed to be drawn. If the line A x be drawn 

 through A so that the angle P Ax shall amount to 42, 

 the rain-drops will reflect coloured drops to the eye. 

 Assuming the line A x to rotate, a cone will evidently 

 be generated, part of which, lying below the horizon, 

 will be invisible. It is the surface of the cone thus 

 generated which is the reflecting surface, and to which, 

 therefore, the rainbow is due. Inasmuch as every 

 colour has a refractive quality peculiar to itself, each 

 drop only represents one tint to the eye. An arc having 

 the breadth of about 2 is sufficient to include all the 



prismatic colours ; 2 therefore is about the breadth of 

 the rainbow. 



Fig. 48. 



The colours of the rainbow are partly due to refraction 

 and partly to reflection, as has been observed. The 

 first effect of light on the drops of rain is refraction, by 

 the operation of which, white light arrives at the pos- 

 terior side of each drop, and is decomposed or dissected 

 into the primitive colours of which it is composed. At 

 the posterior aspect of each drop of rain the dissected 

 colours are reflected unto the eye, and a coloured image 

 U present. -d. 



Such is an explanation of the theory of the primary 

 rainbow besides which, the surrounding rainbow re- 

 quires to be noticed. The secondary rainbow is outside 

 the primary, and is larger than it, but also much fainter. 

 Its angular position is defined by the limits 50 59' and 

 54 9', measured with reference to the axis A x. The 

 secondary rainbow has all the colours of the primary, 

 but less completely defined, and in a reverse order. Its 

 existence may be explained by the statement that it is 

 the result of light twice decomposed, whereas the true 

 rainbow is the result of light only once decomposed. 

 The secondary rainbow, then, is produced by drops of 

 water very far off. 



Inasmuch as any angular elevation of the sun above 

 45 is incompatible with the existence of a rainbow, it is 

 evident that this beautiful meteor can never occur in 

 the south. It may occur, however, either in the east, 

 west, or north. 



As concerns lunar rainbows, they are, as we have before 

 remarked, exceedingly rare, and are very seldom coloured. 

 Nevertheless, in northern latitudes, where the moon 

 shines with a brilliancy unknown to us, coloured lunar 

 rainbows are occasionally seen. 



HALOS AND PARHELIA. These meteoric phenomena 

 are far more rare with us than in more northern lati- 

 tudes, where they are continuously visible for long 

 periods of time, and give rise to phenomena of extra- 

 ordinary beauty. The term hah is applied to a luminous 

 circle occasionally seen around luminous bodies, more 

 particularly the sun and moon, and is partly due to the 

 refraction of light by vaporous water, sometimes in the 

 form of true clouds, sometimes not ; and partly to the 

 properties of light termed diffraction and interference. 

 As respects diffraction, the circumstance has been already 

 announce*', that when light passes through a minute 

 orifice such, for example, as a small aperture punctured 

 in a card the edges of such light bend : this is termed 

 diffraction. The term, interference of light, is used to 

 explain the phenomena of colour, or alterations of 

 luminous condition generally, which result from the 

 assumed jarring impact of luminous waves meeting in 

 different phases of their vibration.* 



Solar halos frequently exist, though unnoticed ; the 

 sun's light being so ppwerfid that the eye of the observer 

 cannot withstand its impressions. By the aid of a sheet 



the Section on Light; articles "Interference o( Lijfht," and 

 " Difffdctiun "ED. 



