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X.— THE USE OF THE MICROSCOPE IN 
THE RUBBER INDUSTRY (EASTERN PLANTATIONS). 
By Herbert Sutcliffe, A.R.C.Sc., F.L.S., F.R.M.S. 
(Read April 19, 1922.) 
Two Plates. 
It may very often happen that certain defects appear during the 
course of the preparation of raw rubber, but such defects only 
become evident in the finished raw product. Usually these 
defects can be avoided by careful preparation, though under some 
kinds of weather conditions at least one defect may appear in the 
finished article in spite of every precaution taken to avoid it. 
Generally speaking the best grades of raw rubber, as far as 
Eastern Plantations are concerned, consist of “ pale crepe ” and 
“ smoked sheet.” The preparation of these in the early stages is 
much the same in both cases, though in the later stages the treat- 
ment of the coagulum is different in each case. 
Certain defects may appear in smoked sheet which would be 
difficult to find in crepe rubber, even though present in quantity. 
By making use of the microscope it is possible to determine 
with more or less certainty what the faults are due to. Usually 
a 16 mm. objective and an X8 or X10 eyepiece will give sufficient 
magnification for most examinations, though in a few cases it may 
be necessary to use a 4 mm. objective. 
Smoked Sheet. 
One of the commonest faults met with in smoked sheet is the 
presence of minute air bubbles which appear as black specks when 
the sheet is held up and examined by direct transmitted light. 
When, however, the observer stands to one side, or turns the sheet 
slightly, the air bubbles, having a different refractive index from 
the rubber, appear as a much lighter patch. This is shown in 
f?g. 1. When, however, the patch of bubbles is examined by 
reflected light it assumes a dull appearance, as though that part of 
the sheet still retained patches of undried rubber. Fig. 2 illus- 
trates this appearance 
These two illustrations only show what is seen when the sheet 
is photographed in the ordinary way. In order to determine 
whether the patch is due to air bubbles or due to moisture in the 
