62 



UNDULATORY FORCES. LIGHT. 



[COMPLEMENTARY COLOURS. 



An immense variety of similar effects of colour to thooo 

 to which we have alluded, is found to occur from a 

 variety of causes. Thus we see the curious play of 

 colours on the surface of melted lead, and heated copper 

 and steel ; the beautiful appearance of mother-of-pearl, 

 fringes in thick glass, which, after heating, has been 

 diplHxl in cold water, <tc. : but we must enter more 

 deeply into the philosophy of the question, by inquiring 

 into the laws of what is termed the inierftrtnee of light. 



This subject may present some little difficulty at first 

 sight : we will, however, by illustration and analogy, 

 endeavour to make it as plain as we can ; and we shall 

 Connect the remarks we have often made in reference 

 to the undulation of an ether, as illustrated by sound, 

 and the circles produced in water when a pebble is cast 

 therein. If two sets of waves are produced on the sur- 

 face of still water, it will be found, that at such places 

 where the waves are of different depths, there they will 

 naturally destroy each other. This is easily seen by 

 casting two pebbles into a pond, at some little distance 

 from each other, and watching the intersection of the 

 waves thus produced. If the two sets of waves are of a 

 similar nature, or in a like phase, their resulting effect 

 will be double that of each taken separately. Now, in 

 the same way as in the cases we have instanced, when an 

 undulation of ether is interfered with by what we may call 

 a half undulation, the result is, that either darkness, 

 that is, absence of light, is produced, or colours varying 

 in their nature, as observed in the spectrum, occur. 



The subject may be still further illustrated by means 

 of a tuning-fork and two glass cylinders. On striking 

 the tuning-fork, and placing it over one of the glasses 

 standing in a vertical position, the air in the glass, ami 

 the vessel itself, will also vibrate. If, however, the other 

 glass is placed so that its mouth is at right angles with 

 that of the vertical glass, all sound will immediately cease, 

 owing to the interference of the two sets of waves of 

 sound thus coming in opposition to each other. The 

 mode of conducting this experiment is represented in 

 Fig. 28, wherein the glasses are found in that position in 

 which the interference of sound can be effected. 



Fig. 28. 



Now, substituting ether for the air (so far as light goes) 

 and water in the two instances we have just selected, 

 the student may imagine two sets of waves thus inter- 

 fering with each other, one of these being represented by 

 the line a in Fig. 29, and the other by the line 6. 



Fig. 29. 



We here find, instead of the depression and altitude of 

 each of the waves being synchronous, or co-incident with 

 each other, they are actually opposed to each other, and 

 that the difference of the position of each curve of the 

 wave is equal to half an undulation. The result is, that 

 at the end of an undulation, the two waves are in differ- 

 ent phases or positions, and interference results there- 

 from. 



We might extend our analogical illustration still further 

 if necessary, and compare tlie different coloured plates 

 produced by the interference of light with the chords 

 which a skilful player on a stringed instrument is enabled 

 to produce by the combination of vibrations of different 

 luir^tha. We trust, however, that our readers havo 

 understood the drift of our remarks, and we shall, there- 

 fore, not pursue our explanations to any further extent. 



To apply these doctrines, we may add, that the colours 



of thin plates, <tc., are the result of the interference of 

 two sets of reflected rsys with each other, wlu-n such are 

 viewed from the surfaces of the glass plate and lens ; and 

 the complementary colours observed by looking through 

 the lens and glass, are the effect of the interference of 

 the transmitted rays with each other. 



We may here explain more fully than we have hitherto 

 done, the meaning of the term ' ' complementary colours. " 

 If the eye watch one colour intensely for some time, 

 on removing this from the sight, an image of the object, 

 of an entirely different colour, will be perceived. A sim- 

 ple way of trying this, is by placing a red wafer on a 

 sheet of paper. After looking at this intensely for a few 

 minutes, the eye will perceive a green image of the wafer, 

 if the sight be directed to another place. Sheets of deeply 

 coloured paper answer a similar purpose ; and thus, if a 

 person look intently for any length of time on the back 

 of a book or other object of a deep red or yellow colour, 

 a spectral image will be observed, of a green or violet 

 colour respectively. 



The colours so observed are complementary to those 

 of the object ; or, in other words, are those which, with 

 that of the object, would together produce white light. 

 For the information of the student, we append a table of 

 the different colours of the spectrum, or of any object, 

 and of their corresponding coinplementaries. 



Colour of Object. 



Violet . 



Its Complementary. 



Yellow. 



Green Red. 



Yellow Violet. 



Orange Blue. 



Red Green. 



A very interesting application of this property of 

 colours is often made in economic and commercial affairs. 

 Coloured fabrics are generally arranged in covers, or 

 papers of such colours as are complementary to the 

 articles they enclose. Thus a green silk or cotton fabric 

 appears of a far richer colour when wrapped with, or 

 placed on, a sheet of reddish paper, than when placed on 

 a white one. By judiciously choosing such arrangements 

 the saleable nature of the article is often enhanced. This 

 is another instance of scientific laws being often applied 

 by persons ignorant of their existence, who, however, by 

 long practice have blindly arrived at truth. Great taste 

 is often exhibited by ladies in the arrangement of the 

 colours of the different parts of their dress, in their un- 

 wittingly following the principles we have laid down. 

 We may here, perhaps, be permitted to become the 

 apologists for some of the fair sex, who do not seem to 

 bo characterised with correct taste in this respect. Wt> 

 do so by stating, that it frequently occurs that such have 

 not the optical power of judging of such matters, and 

 therefore are not to be blamed for what, at first sight, 

 would appear to indicate an absence of refinement or 

 judgment. We shall enter more fully into this question 

 hereafter, when we consider certain physical defects in 

 the construction of the eye, which incapacitate many 

 persons, more or less, in discriminating coloured objects. 

 The law of contrasts of colours, as applied to dress and 

 to commercial purposes, although of great interest, falls 

 scarcely within the scope of our plan. We have, how- 

 ever, much pleasure in recommending the perusal of 

 a work entitled Colours and their Contrasts, by M. 

 Chevreul, a French chemist ; in which the subject is 

 treated in a complete and masterly manner, and in which 

 all the laws of complementary colours are fully entered 

 into, and their applications judiciously pointed out. 



DIFFRACTION. 



IN ronnection with the interference of the undulations 

 of lixht, we must now refer to the phenomena of dijf'i 

 tton, or the production of coloured fringes by means of 

 .sp.vial an acting on a diverging ray of light. 



A very easy mode of nlworving the effects of the dif- 

 ielion of light, is by admitting into a dark room a ruy 

 of light through a short-focus lens, and placing a knitting- 



