( selzxayir } 
more owing to double diffraction and also to refraction, so 
that it is necessary to be most particular for all observations 
to be made on isolated scales so far as they relate to these 
structural colours, 7. e. to colours that differ from the pigment 
found within the scale. 
In closing I must refer to my investigations on the colours 
of the scales we have been considering. I can of course only 
refer to a few more or less typical species, or to those that 
differ from the usual rule. It will no doubt be understood 
after my previous remarks that in all cases I am dealing with 
isolated scales only; in many instances I have removed the 
upper ribbed chitinous surface of the scale sack, and frequently 
the lower surface, my object being to find the contents of the 
sack. 
Plebeius argus under transmitted light has scales (blue under 
direct light) that are quite colourless, some being 
the very palest straw colour. On removing the 
ribbed upper wall of the sack a very fine granulated 
surface is disclosed that is absolutely colourless and 
transparent. 
P. sveversii, a decidedly blue species, is under transmitted 
light straw colour, as also are the thick hair-like 
scales; with the ribbed chitine removed the sack 
is granulated, the pale yellowish granules lying in 
fine lines so as to coincide with the ribbing, the 
hair-like scale was likewise granulated but with the 
chitine removed appeared to be quite colourless. 
P. eversmanni has its blue scales reddish buff under trans- 
mitted light; with the ribbed wall removed, the 
sack is roughly granulated, the pinkish granules not 
being confined to the lines of the ribbing. 
Polyommatus icarus is pale lemon yellow under transmitted 
light; with the upper wall removed a very finely 
granulated surface is disclosed, which under a 
moderately high power is found to consist of minute 
yellow granules that follow precisely the lines of the 
ribbing. 
P. thetis, with its clear and beautiful blue, becomes under 
