Iridescent Colours. 153 



proved to me that, almost without exception, the colours of 

 natural iridescent objects are due to interference produced 

 by thin plates. In order, therefore, to render clear the 

 principles on which the method I propose is founded, I will 

 briefly refer to certain fundamental facts in connection with 

 colour production by thin plates, and for this purpose 

 will select a thin film of mica, which, with light at perpen- 

 dicular incidence, appears red, iridescent red. If, now, 

 this plate be inclined so that the light falls on it at a more 

 oblique angle, it is, of course, reflected at the same angle, 

 and now appears orange, and if the plate be still further 

 inclined, the reflected light appears yellow, then yellowish 

 green, green, and bluish green, and if the light were not too 

 copiously reflected from the first surface to allow of per- 

 ceptible interference by further inclination of the plate, all 

 the colours of the spectrum in their proper sequence might 

 be observed. The same results, but much more vividly, 

 may be seen in these crystals of chlorate of potash. Thus, 

 we see that by rendering the incident light more and more 

 oblique, the reflected light changes from a lower to a higher 

 tint, that is, Irom the red towards the violet end of the 

 spectrum. And this is what occurs in the case of all 

 iridescent bodies, as the incident light becomes more oblique 

 the colour changes to the tint above it in the spectral order, 

 so that, if we know what colour any such object appears 

 when seen at a certain angle, we can infer what colour it 

 will change to on varying the incidence. This beetle (Sagra 

 purpurea), for instance, is red at perpendicular incidence, it 

 will, therefore, appear orange yellow and green when 

 examined by successively increased obliquity of light 

 And the same is true of all other iridescent red objects. If 

 the object at perpendicular incidence be green, as in the 

 case of this beetle (Bupristis), it will become blue and 

 then violet as the incidence is increased. We thus see 

 that an iridescent object varies in colour, simply because it 



