IRIDESCENT COLOURS. 282 
of the operation, as if the screens are sound and the drive conducted with skill and patience 
scarcely any locusts can escape being driven into the pits, The tin rim obviates the necessity 
of the pits being dug deep, 2 or at more 3 feet being sufficient. The rims used were 4 feet by 
2 feet, an edging of 24 inches of tin on the ground surface round the pit, and the same width 
on the inside edge of it. 
“T recret that [knew of this plan too late to provide sufficient screens for general use} 
I believe that this system will be found most efficacious, and feel confident that had we been 
prepared with this apparatus, the work of destruction would have been carried on with less 
trouble and with better results.” 
In Jhang, according to a crop report issued in June, 1891, twenty thousand 
maunds of locusts had up to that time been destroyed. 
The above comprises all detailed information which has reached the Museum 
on the subject of what was done in the Punjab in the Spring of 1891, but 
numerous incidental notices have been received of the work of destruction 
which seems to have gone on systematically in all districts where young 
locusts hatched out. 
With regard to what was done in Sind and Rajputana, where egg-laying 
also went on, little fresh information has been obtained, but the people seem, 
as usual, to have done what they could in the way of destroying the young 
locusts by driving them into trenches. 
In the case of the measures taken in districts that were only visited by 
flights, no fresh information has been received, but the system which has proved 
so successful of driving the insects off the crops is believed to have been 
universally adopted by the cultivators. 
Ki. C. Cores, 
Deputy Superintendent, 
Indian Museum, Calcutta. 
Sth June, 1892. 
ON IRIDESCENT COLOURS AND A METHOD OF EXAMINING 
IRIDESCENT OBJECTS, BIRDS, INSECTS, MINERALS, &c., SO 
AS TO ENSURE UNIFORMITY IN THEIR DESCRIPTION. 
By ALEx. HODGKINSON, M.B., B.SC.” 
ON taking a general survey of coloured objects, whether natural or artificial, 
we become aware of the fact that whilst the colours of some remain unchanged 
as regards tint, whatever their position in relation to the incident light, the 
tint of others varies with every alteration in their relationship to such light 
source. We thus see that, so far as their colours are concerned, all bodies may 
be arranged in two groups according as their colours change or do not change 
in tint as their angular relationship to the light varies, Nor is this classifica- 
tion entirely an artificial one, since, as will shortly be seen, though this change 
* This Paper appeared originally in the Memoirs and Proceedings of the’ Manchester 
Literary and Philosophical Society, Vol. V, Part IT, ; 
16 
