250 APPENDIX. 



a definition of the appearance produced by direct liglit, 

 used in the simplest and purest manner, viz. with the 

 Achromatic Condenser and parallel rays from a Mirror 

 or Right- Angle Prism, and have necessarily omitted 

 any allusion to the varied appearances to be produced 

 by changing the direction or character of the light, 

 although I have used these plans in determining struc- 

 ture, my object being to give such a description as may 

 enable the observer to identify the particular species. 

 My definitions of structure may clash with the views 

 of some observers ; they have been determined after 

 careful observation and after many experiments. 



I am convinced that the scales consist of two mem- 

 branes ; I have seen them partially separated. I have 

 satisfied myself that the two exposed surfaces are totally 

 dissimilar ; that in all cases the under surface, or that 

 nearest the body of the insect, is corrugated ; that in 

 all cases the upper surface is much less uneven, and 

 in many is so slight in its irregularities that it may be 

 described as smooth, whilst I attribute the beaded 

 appearance, so often spoken of and so easily produced, 

 as due to the combination of the external corrugated 

 structure of the lower membrane and the internal 

 structure of the upper membrane. 



There will be found two illustrations of the scale of 

 Lepidocyrtus curvicolUs. I am unable to explain the 

 reason for the difference in the size and appearance of 

 the scales. They are taken from insects, entomologically 

 speaking, the same, and yet I have never found the two 

 sorts of scales on the same insect, and have never found 

 the insect with the large scale in the same habit at as 

 the one having the smaller and more common form. It 

 is the large scale which is so valuable as a test of the 

 quality of an object-glass, from the facility with which 

 it displays both the spherical and chromatic aberra- 

 tions. 



I have not given illustrations of the scales of some 

 of the smaller Lepidocyrti. I have examined all but 

 those of L. ceneiLS ; they are all very minute, but under 



