108 THE entomologist's record. 



on. Substances which are thus able to select certain light waves for absorp- 

 tion, and to return (reflect) others to our eyes, are ordinarily termed pig- 

 ments, and the fact that scales of lepidoptera contain largely substances 

 that can do this causes us to term the colours thus produced pigment 

 colours. 



Besides this, if white light passes through various media of different 

 degi'ees of density, the light is bent out of its original path and is said 

 to be refracted ; at the same time the light is often not only refracted, 

 but its constituent waves are largely separated, and the light is in this 

 way decomposed. Colours thus produced are termed refraction colours. 

 The colours produced when light passes from air into a glass prism (a 

 denser medium) are familiar to all. 



Tliin as the walls of a soap-bubble are, yet they have some thickness, 

 and it is evident that if light falls upon its filmy wall and part of the light 

 is reflected from its outer surface, whilst a part also enters the film and 

 is reflected from its inner surface, that, in spite of the thinness, the 

 vibrations of that portion of the light- waves reflected from the inner 

 surface will fall behind those reflected from the outer surface. These 

 two sets of waves then do not fall synchronously, but interfere with 

 each other. As a result of this interference, the light is broken up 

 and interference colours are the result. Everyone has seen the inter- 

 ference colours of the soap-bubble. 



Light travels in straight lines, yet it has a slight power of bending 

 round the edges of bodies. Light and shadow are rarely absolutely 

 defined l)y a strongly-marked line. To this bending of light- waves 

 the term diffraction is applied. Light passing through very fine slits 

 is diffracted, and scratched or striated surfaces act similarly on light. 

 Diffraction breaks uji the bent part of the ray into its component 

 parts and, dispersing the waves, gives on the edge of each bright space, 

 between the slits or striations, a fringe of colour. Interference 

 l)henomena are sometimes closely connected with diffraction. The 

 beautiful colours of mother-of-pearl are in part due to the finely 

 striated surface thereof, for a wax impression of the part will give the 

 same effect. 



It is necessary to bear in mind one other thing, viz. — that the com- 

 bination of differently coloured lights produces far different effects 

 from the combination of differently coloured pigment powders. That 

 coloured light which, when combined with another, forms white light, is 

 termed complementary ; thus, blue light and yellow light combined form 

 white light ; red and greenish-blue, orange and Prussian blue, yellow and 

 indigo-blue, and greenish-yellow and violet are also com})Iementary 

 colours. If a pigment be prepared by grinding a blue and a yellow 

 substance together a green is produced, not white as would be obtained 

 from a combination of blue light and yellow light, because, in this case, 

 we do not add together the colours which each reflects alone, but we add, 

 by uniting the })igments, their absorptive powers. A fuller explana- 

 tion of this can of course be obtained from any text-book on light. 



These short notes may, perhaps, help the student whose knowledge 

 of physics is nil, to grasp the general principles involved in this 

 discussion, but it is evident that only those who already know, or will 

 read up the technicalities involved from an authoritative text-book, can 

 hope to do so with success. At any rate, these notes should be suffi- 

 cient to enable readers to distinguish between colours due to the 



