May i6, 1878] 



NATURE 



59 



The first, under all circumstances, is a molecule or 

 series of molecules in vibration, and the quality of the light 

 depends upon the vibration either inherent in the mole- 

 cule or dependent upon the quality of the energy which 

 sets it in vibration or controls the vibration. 



The second is the ether, which does for light what our 

 atmosphere does for sound. Competent to transmit vibra- 

 tions of all lengths without loss of energy, it behaves 

 with perfect fairness, so to speak, to light of all kinds. 



The third, the receiving instrument, in our case is the 

 eye of the artist above all things, but not to the exclusion 

 of everything else, because every object which reflects 

 light must receive it first, and sometimes important modi- 

 fications are brought about in the act of reflection. To 

 , mention two instances : — white light from the sun falls on 

 a leaf, but leaves appear green by the light which they re- 

 flect, and this transformation is the result of molecularwork. 

 The light of the moon is yellow in comparison with sunlight 

 for the same reason, and the difference between sunlight 

 and moonlight effects has its origin in the lunar molecules. 



We call the light of the sun white, and much of the 

 action of light may be studied by supposing this light 

 to be a simple thing, by which I mean non-compound. 

 A hole in a shutter through which the sun shines will 

 convince us that light travels in straight lines. The 

 idea of a modern novelist that light can travel spirally 

 through a key-hole is not based on fact. 



It may happen, too, if there be a brightly illuminated 

 object outside the hole in the shutter, that another point 

 not to be neglected will be illustrated. Because light 

 does travel in straight lines the various rays coming from 

 the different parts of an object and passing through a 

 small aperture will build up an inverted image of it on 

 the other side of the aperture. The most obvious bearing 

 of this in artist's work is that if the sun shines through the 

 narrow apertures left by leaves in a thick wood, we shall 

 have images of the sun on the ground ; the shadows of the 

 leaves will be dominated by circular intervals, and the 

 higher the leaves the larger these circles will be. During 

 the eclipse of 1 870 the images of the delicate crescent of 

 the Sim thrown on the ground through the orange and 

 olive trees were most perfect in form, and produced a 

 Strange effect never to be forgotten. 



The laws of reflection can also be studied without any 

 higher knowledge of the properties of light, and the dif- 

 ference between " specular reflection," the case in which 

 light is reflected as in a mirror, and "scattering," in 

 which, in consequence of the roughness of the surface on 

 which it falls it is thrown off in all directions, will be at 

 once recognised. 



Water is the great reflector employed by the artist. 

 Take, for instance, No. 643 in this year' s Academy. If a 

 painter will imagine a vertical plane passing through the 

 object reflected — say a hill-top, and his eye ; and plot a 

 section with the height of his eye and the hill-top above 

 the water and the distance between them roughly to scale ; 

 and if he will further recollect that the lines which con- 

 nect the reflecting point of the water with his eye and 

 the hill-top must make equal angles with the water level, 

 he will find all he needs to insure correctness. In a 

 picture taken, e.g., with the eye at a, in the annexed 

 woodcut, he will not include the distant hill-top at y, 

 while with the eye at b he would do so. 



In tranquil water the same consideration determines 

 the locus of the reflection of the sun or moon. But if 

 there be scattering, there will be a wake. The above 



Fig. I. 

 reasoning will show that it will be absolutely incorrect to 

 throw this wake athwart the picture ; nevertheless, this 

 has been done, and by artists of the highest celebrity. 



It is, perhaps, in the case of reflection of light by the 

 poor moon that the modern artist comes to the greatest 

 grief ; and yet the only peculiarity in this case of reflection 

 of light is that we are dealing with reflection from a 

 spherical body which changes its place with reference to 

 the light source and our eyes. If an artist would amuse 

 himself any evening with his children in imitating these 

 conditions with a lamp and some oranges he would never 

 make another mistake. We should have moons painted 

 with discretion instead of ct, discretion, and I fancy the 

 "balance of the picture" would be found to be much 

 less frequently disturbed by scientific accuracy than is 

 generally imagined. I have known an artist to defend 

 a crescent moon directly opposite a setting sun on the 

 ground that if the moon had been painted full this 

 much-prized "balance of the picture" would be entirely 

 upset ; and yet when it was suggested that the effect of 

 two moons should be tried the idea was scouted as 

 ridiculous— why, I could not understand from the stand- 

 point taken by the artist. 



Five minutes' reading of any elementary astronomy is 

 all that is necessary to show that when the moon is on 

 one side of the horizon, say east, and the sun on the 

 other, say west, the observer must be between them, and 

 that therefore the moon' s reflecting side in its entirety 

 must be turned towards him. It will not want even this 

 amount of reading to convince him that if a sunset is 

 painted as in Fig. 2, by no possibility can a globe at 



Fig 2. 



a be lighted up, as shown. Nor can it be lighted up 

 by the sun as shown at b, because the sun must be 

 somewhere on the dotted line. If the moon is drawn at c 

 the illumination must be symmetrical with reference to 

 the line joining the moon and sun, and the nearer the 

 moon is apparently to the sun, the more delicate must the 

 crescent be. 



Enough for the present on this subject. There is, how- 

 ever, one other point worth notice, which, although it 

 is more astronomical than physical, I may perhaps be 



