COLOUR. 17 



insect's beauty, and shows that the glittering points are the 

 edges of innumerable pits or depressions with which the entire 

 surface is covered. It is not, however, until a power of some 

 two hundred diameters is employed that the real nature of these 

 points and the cause of their changeful beauty are shown. Not 

 in the minutest spot is there a smooth portion, but the elytron is 

 completely covered with an elaborate series of six-sided pits or 

 shallow cells. In fact, when viewed with a high power and 

 a favourable light, the upper surface of the elytron looks very 

 much like a honeycomb, the cells with which it is covered being 

 perfectly hexagonal. The sides, however, of the cells are not 

 equal as in the honeycomb, but two opposite sides are twice the 

 length of the others, so as to elongate the cells without altering 

 their hexagonal form. The shape of the cells is best seen in the 

 creamy spots, as the dazzling blue and green of the other parts 

 rather distract the eye. 



Each of these cells has its surface furnished with countless 

 dented lines set parallel to each other, and producing, by 

 means of their power of decomposing the light, the wonderful 

 effects of colour wdiich have been mentioned. The gorgeous 

 metallic plumage of the humming-bird's breast owes its splen- 

 dour to a similar cause, and so does the changeful purple of our 

 " Purple Emperor" Butterfly. The opal and the pearl also owe 

 their colours to this system of parallel lines, which can now be 

 produced artificially on almost any substance. Delicate parallel 

 lines can be ruled on steel or glass, giving to them a flickering 

 brilliance of colour that seems quite foreign to the material, 

 which, indeed, appears to be of secondary importance. As 

 long as the lines are there, the material seems to be of little 

 consequence ; and if a piece of soft cobbler's wax be pressed on 

 the ruled glass or steel, the lovely iridescence will at once be 

 evident on its surface, despite the apparent incongruity of the 

 dull black material and the splendid colours which play over it. 



Such are some of the revelations of the microscope with 

 regard to the colouring of the velvet-like surface of the elytra. 

 Now, to the unaided eye, the vivid green and azure of the head 

 are much more brilliant than the same hues in the elytra, while 

 they are not so bright when placed under the microscope. The 

 reason is this. The surface of the head is covered with a double 

 series of wrinkled folds, which are so large that they can he 



c 



