HARDWICKE'S SCIENCE-GOSSIP. 



CHAPTERS ON COLOUR; 



By S. A. Notcutt, jun., B.A., B.Sc. 



No. I. 



THE natural starting-point in this subject is the 

 colour of ordinary bodies and pigments which 

 we see by the aid of external light, usually white 

 light. This light, on passing through a prism is, we 

 know, split up into the prismatic colours, forming 

 what is called the spectrum ; and thus we learn that 

 sunlight is composed of rays of light of almost every 

 different colour. On holding a piece of red glass in 

 the path of the rays as they issue from the prism, the 

 green and blue portions of the spectrum disappear, 

 leaving little else than red, for the red glass absorbs 

 the green and blue rays. 



Similarly, blue glass would absorb the yellow and 

 other rays, and, in fact, with various glasses and 

 coloured solutions, we can absorb any particular set 

 of rays we like, and in each case the rays that 

 remain after this process of absorption constitute the 

 particular colour of the glass or solution. 



This is the case with transparent coloured bodies. 

 In the case of an opaque body, such as a pigment, a 

 portion of the light falling on it is reflected at the 

 surface, and as a rule is not changed in colour ; but 

 another portion penetrates the substance, and since 

 the internal structure of such a substance is always 

 irregular, this portion of the light is soon reflected 

 back ; however, in passing and repassing through the 

 superficial layer of the substance, it suffers absorp- 

 tion, and hence issues forth as coloured light, the 

 colour being what we recognise as the colour of the 

 body. 



In a leaf a very thin layer of the colouring matter, 

 chlorophyl, is sufficient to absorb all the orange, blue 

 and violet rays contained in the incident light; hence 

 the light reflected back from the interior of the leaf is 

 without these rays, and since the remaining rays 

 together constitute a green light the leaf looks green. 



" Thus," says Tyndall, " natural bodies have 

 showered upon them, in the white light of the sun, 

 the sum total of all possible colours ; and their action 

 is limited to the sifting of that total, the appropriating 

 from it of the colours which really belong to them, 

 and the rejecting of those^ which do not. We may 

 therefore say that it is the portion of light which 

 they reject, and not that which belongs to them, that 

 gives bodies their colours." 



It is sometimes convenient to be able to compare 

 by means of a diagram the light transmitted by two 

 different media. This is usually accomplished by 

 taking a rectangle to represent the solar spectrum, 

 and in it drawing a curve, the ordinates or distances 

 measured upwards for any point of the curve repre- 

 senting the amount of light which is transmitted of 

 that particular part of the spectrum. 



On comparing the diagram of the light reflected by 

 green pigment with that of the green light from 

 vegetation, we find a considerable difference (Figs. 2 

 and 3). We see that besides a quantity of yellow 

 and green being transmitted by the foliage green, 

 a portion of the extreme red is also transmitted. 

 If we therefore cut off the yellow and green light 

 coming from foliage, we should expect it to appear 

 red, and this is seen to be the case on viewing 

 a garden or field through 'glass stained a deep blue 

 with cobalt. In sunlight a piece of yellow glass 

 should be added, to cut off the extreme blue and 

 violet. 



A sunny landscape viewed through these two 

 glasses presents a curious appearance : green trees 

 and plants are a red colour, the sky is greenish-blue, 

 the clouds purplish -violet, and anything orange 

 appears blood-red. The absorption diagram of these 

 two glasses shows that they cut off almost all the green 

 light furnished by leaves, but transmit the bluish-green 

 rays which leaves do not furnish. 



An interesting phenomenon depending on absorp- 

 tion is that known as dichromatism, which is the 

 variation in the apparent colour of an absorbing 

 medium when different thicknesses are used. Thus 

 the colour of a solution of litmus enclosed ;in a 

 wedge-shaped glass vessel, varies from blue at the 

 thinnest part to red at the thickest. Chromium 

 chloride varies from green to red ; potassium perman- 

 ganate from purple to blue ; reduced haemoglobin 

 from green to purple. Several thicknesses of yellow 

 glass appear red. 



This phenomenon depends on the principle that if 

 a certain thickness, say one centimetre, of a medium 

 absorbs a certain proportion of the rays, then the 

 same proportion of the remainder of the rays will be 

 absorbed on passing through another centimetre of 

 the medium. In the case of litmus, suppose that in 

 the incident light there are one hundred blue rays to 

 every ten rays of such wave length, that they are 

 partially transmitted by the solution, whilst rays of 

 any other refrangibility are completely absorbed ; 

 then after passing through an extremely thin layer, 

 the emergent light will be a deep blue, the proportion 

 of blue to red being ten to one, and the red being 

 thereupon scarcely noticeable. Now suppose that 

 each millimetre of the solution absorbs one-tenth of 

 the red and one-half of the blue rays, then after 

 passing through 1 mm., the light will contain nine red 

 rays to every fifty blue, and after passing through 

 successive millimetres of the solution, the proportion 

 of red to blue rays will be as the table below (taken 

 from Glazebrook's " Physical Optics "), from which it 

 is apparent that the relative intensity of red to blue in 

 the emergent light has altered from a proportion of 

 1 to 10 to one of more than 3 to 1 as the thickness, 

 has been increased to 6 mms., and the light is finally 

 a reddish-purple, which may be made quite red by 

 increasing the thickness sufficiently. 



