200 



NA TURE 



[June 30, 1887 



artist that when he gets his picture, No. 137, back from 

 the Academy, he should carefully take out the moon, 

 because its presence shows that the sun is not setting, 

 and if the sun is not setting, then the colours on the 

 clouds are false ; without the moon the picture would be 

 entirely satisfactory to a keen observer of natural pheno- 

 mena, assuming the sunset hour to be approaching, for 

 the colours of the clouds are most beautiful. There can 

 be no doubt, we think, that the study of cloud-forms is now 

 not so neglected as it used to be. Some of the forms of 

 clouds in many of the pictures this year indicate a 

 very close attention given to them and to their 

 floating changes by the artists ; witness Nos. 459, 945, 

 and 28. 



The forms of water, too, and the illuminations of a 

 water surface, are admirably represented in many pictures 

 this year. The tracery of the water surface could scarcely 

 be more exquisitely shown than in No. 1 18, the picture we 

 have already referred to as possessing a most unfortunate 

 moon. The little roller stretching nearly across 630 

 seems absolutely approaching us, while the view of the 

 opening sea with the solitary sail in No. 659 positively 

 makes one feel as if one were on a Cunarder, revelling in 

 the fresh free air. 



Although we have pointed out much that we hold to 

 be very excellent in the way of sky and sea colour, we 

 cannot help thinking that the chalkiness is on the in- 

 crease : we never remember to have seen before so many 

 seas and skies resembling chalk and water. This is an 

 effect really very rarely observed in the sky, and not 

 often in the sea, excepting close in shore, and in the 

 absorptive properties of water we have a very good 

 reason why it should not be so. Let the reader look at 

 223, 254, 274, 353, and 419. 



There is no help for it, we must say a few words about 

 rainbows. Why should an artist who has never seen a 

 rainbow, or, if he has seen one, been so careless as not to 

 take the trouble to look at it or to observe the order of its 

 colours, why, we say, should he take the trouble to paint 

 it ? why does he not leave it alone ? As most people 

 know, the colours in the rainbow are regulated by a 

 definite law — that is to say, the order of colours is the 

 same. If we have red on one side of the bow, then we 

 have to pass through orange, yellow, green, and blue, till 

 we reach the violet on the other side. But in the two 

 rainbows portrayed this year in the Academy, this order 

 is not at all followed. One of the artists preferred to put 

 the green between the yellow and the red, and the otlaer 

 thinks he has found a more excellent way. The result is 

 that these gentlemen present us, under the guise of a 

 rainbow, with a phenomenon which no mortal has ever 

 seen or ever will see. 



It is a matter, we should have thought, of general 

 knowledge that a rainbow is caused by the action of rain- 

 drops upon the light which impinges upon them. In the 

 annexed woodcut, we may imagine the beam s I a sun- 

 beam entering a drop of water, a section of which is 

 shown, A 1 1'. The beam is refracted on its entrance into 

 the drop at l, is reflected at A, and is again refracted on 

 emerging from the drop at 1'. The light which originally 

 entered the drop in the direction S I leaves it in the 

 direction l' M, and the eye, to see the raindrop, must be 

 along the line i' M. The observer, therefore, must ob- 

 viously have his back to the sun, and the bow will appear 

 to be at some distance above the horizon. If the sun- 

 light could not be broken up into various colours by 

 refraction, the bow would not appear coloured ; but as 

 refraction, which has two chances of working, does break 

 up white light into its constituent colours, the emerging 

 beam 1' M is coloured, and the order of colours must 

 necessarily be the same as that which is observed by 

 means of an ordinary prism or lustre. We begin with the 

 warmest colours, the reds and oranges, outside the bow, 

 the inside of the bow being formed of the cooler colours. 



blue and violet. This rainbow, which is the one observed 

 under ordinary conditions, is formed by means of the 

 rays of light falling on the outer portions of the drops, 

 and is called the primary bow ; as we see, the light suffers 

 two refractions and one reflection in the drop. 



There is another rainbow seen, when the conditions are 

 entirely favourable, outside the former or primary one. 

 This is called the secondary bow ; it depends for its 

 existence upon the light which falls nearer the front sur- 

 face of the drop, a condition of things shown in the next 

 woodcut, in which S i represents the light which is re- 

 fracted at I, reflected internally at A, reflected internally 



Fig. I. — Path of light in producing primary bow. 



again at B, and refracted at i', and sent along the path I M 

 to the eye. The eye, which receives it therefore in the 

 line r M, receives it after two refractions and two internal 

 reflections. 



In this secondary bow the order of colours is reversed. 

 We get the warm colours, the reds and the yellows, inside, 

 and the blue and violet outside, so that when the primary 

 and secondary bows are both seen, the two reds are 

 together, and the two blues as far apart as they can be. 



When we talk of the spectrum colours in the rainbow, 

 it must be always understood that the appearance is not 

 that assumed by an absokitely pure spectrum, for the reason 



Fig. 2. — Path of light in producing secondary bow. 



that the sun, the light of which is in question, has a disk 

 of visible magnitude, and that the ray of hght coming 

 from each point of the sun is competent to produce a 

 rainbow. Hence the rainbow is a very composite thing, 

 and really consists of millions of spectra overlapping. 

 The more the overlapping is intensified, the more the 

 pure colours disappear, and hence it is that the rainbows 

 seen when the sun is behind a cloud are very often seen 

 without any trace of colour. The more the light proceeds 

 from a large surface, the dimmer and less clear will the 

 rainbow become, until at last it fades into invisibility. 

 An artist in painting a rainbow, then, has to consider 



