240 



EECEEATIVE SCIENCE. 



pictorial art, we have to consider the value 

 of lessons to all classes of students ; for M. 

 Chevreul pursues the subject through the 

 several ramifications of painting, staining, 

 dyeing, printing, the disposition of draperies 

 and furniture, and the arrangement of na- 

 tural objects, as in landscape and flower 

 gardening. 



Those who desire to compass the scien- 

 tific principles of harmony and contrast, will 

 do well to read with close attention the in- 

 troductory chapters of M. Chevreul's book. 

 We can but give a general idea of its utility. 

 The law of simultaneous contrast is shown 

 by a variety of experimental demonstrations, 

 all of which are readily intelligible, and are 

 of so pleasing a kind that the most casual 

 student soon finds himself studying with an 

 interest which expands and deepens at every 

 fresh example. A ray of solar light is com- 

 posed, as most people know, primarily of 

 three rays — ^blue, red, and yellow — and inde- 

 terminately of various coloured rays, which, 

 being distributed in groups, are termed red, 

 orange, yellow, green, blue, indigo, and violet 

 rays. "When light falling on a body is com- 

 pletely absorbed by that body, as in falling 

 into a deep cavity, then the body appears to 

 us black, and it becomes visible to us only 

 because of its contiguity with surfaces which 

 reflect or transmit light. ISTo bodies are 

 known to be perfectly black ; that is to say, 

 no bodies exist which absorb light entirely ; 

 and it is just by their reflecting a little white 

 light that we are enabled to discern their form 

 in relief, like other material objects. When 

 solar light is reflected by an opaque coloured 

 body, there is a reflection of white light, and 

 also a reflection of coloured light ; the latter 

 being accounted for by the fact, that the body 

 absorbs or extinguishes within itself some of 

 the coloured rays wliile reflecting the others. 

 A moment's thought will bring us to the con- 

 clusion that, as the absorbed rays and the 

 reflected rays must be of different colours, so 

 their renunion produces white light again. 



In brief, it may be said, with regard to the 

 physical composition of solar light, that if the 

 whole of the coloured light absorbed by a 

 coloured body were reunited with the whole of 

 the coloured light which it reflects, white light 

 would be the result. The phrase, " comple- 

 mentary colours," is used to express this 

 property of two oppositely-coloured lights. 

 Thus, we call red the complementary of green, 

 because green, being combined blue and yel- 

 low, makes with red the complement of the 

 prism. In the same manner blue is the com- 

 plementary of orange (yellow and red) ; and 

 yellow is the complementary of dark violet or 

 purple (red and blue). With secondary and 

 tertiary, as well as with primary tints, the 

 same rule holds good ; greenish yellow being 

 the complementary of violet, and orange- 

 yellow of indigo. Such are the leading facts, 

 which, roughly laid down, will prepare the 

 student for the consideration of many inte- 

 resting phenomena, and for the pursuit of 

 many instriictive experiments. 



The simultaneous contrast of colours may 

 be shown in several ways. If two stripes of 

 the same colour, but of unequal tints, be 

 placed together, so that the edges meet, we 

 shall detect, if the stripes be not too broad, 

 certain modifications, firstly affecting the in- 

 tensity of the tints, and, secondly, the optical 

 composition of the colours. To these modi- 

 fications, inasmuch as they cause the colours 

 to appear, when looked at together, more dif- 

 ferent than ihey really are, is applied this 

 designation, " the simultaneous contrast of 

 colours." The contrast of tone may be treated 

 as a separate phenomenon, and we may gain 

 some idea of this separate phenomenon by 

 means of a simple, but very curious, series of 

 experiments. Two square pieces of unglazed 

 paper of a large size {a a), being coloured a 

 clear gray, by a mixture of chalk and any 

 black pigment, are to be affixed, at a space of 

 rather more than half the width of either, 

 on a piece of unbleached linen. Then two 

 squares of paper of the same size (b b), 



