Colouring in Birds and Insects. 5G5 



shown in PL IV. fig. 7 (No. 6). There is in this case no 

 such minute linear structure as in the case of Morplio alga ; 

 and as here the phenomenon is clearly a case of " surface- 

 colour," so it is highly probable that the same cause is 

 effective in the case of Morplio. 



Many other specimens were subsequently examined, but 

 all fell into one or other of the two classes typified by these 

 two. 



Diamond Beetle. 



If a specimen of the beetle popularly known as the Diamond 

 Beetle be examined with a low power under the microscope, 

 the bright green dots on the wing-case are seen to consist of 

 depressions from which spring brilliant and exquisitely 

 coloured scales ; the colours varying throughout the range 

 of the spectrum (green, however, predominating). 



The colours exhibited by these scales are so vivid and 

 varied, and the changes so rapid with varying incidence, 

 that it was at once evident that the effect must be due to 

 diffraction from regular striations, which were accordingly 

 looked for under a magnification of about 1000 diameters. 

 There were occasionally faint indications of striated structure, 

 but so uncertain that if other indications had been less 

 decided it might have been concluded that some other cause 

 must have been effective. But on putting the microscope- 

 out of focus a moderately pure spectrum was observed, and 

 by measuring the angles of incidence and diffraction of the 

 various colours, the "grating" space could be determined, 

 and was found to be of the order of a thousandth to a two 

 thousandth of a millimetre. 



The specimen was next examined by reflected light* and 

 the striations at once appeared, the count of the striations 

 giving numbers agreeing very well with the calculated values. 

 Frequently a single scale showed two or even three series of 

 striations, giving corresponding spectra, in three different 

 directions. Another important feature of these "gratings" 

 was shown in the fact that the light is all concentrated in a 

 single spectrum, showing that the striations must have an 

 unsymmetrical saw-tooth shape f. 



* The observation is somewhat difficult on account of the very small 

 •working* space when using high powers. 



t It may be noted that the objection that the colours of birds and 

 insects cannot be due to diffraction on account of the equalizing- effects 

 of the varying- angles of incidence and diffraction, would not apply if 

 the striations are so fine as to give practically a single spectrum extending 

 over a range of 45°. 



