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microscope of this description before I found that the prin- 
ciple could be applied to microscopes of the ordinary con- 
struction. 
It may be interesting to know what is the exact loss of 
power in the object-glasses by submersion in water when 
enclosed in the protecting tube. I give the results in the 
case of three objeet-glasses belonging to my microscope : 
No. 1.—Focus in air, ‘$2 inch. 
water (with submersion tube on), *95 inch. 
Difference, 13 inch. 
No. 2.—Focus in air, ‘27 inch. 
a water (with submersion tube on), *31 inch. 
Difference, -04 inch. 
No. 3.—Focus in air, ‘11 inch. 
=] water (with submersion tube on), ‘14 inch. 
Difference, ‘03 inch. 
The differences in these cases are of little practical conse- 
quence,! and are very much less than when either surface of 
the object-glass comes directly in contact with the water. 
In the case of a plano-conyex lens, if the plane surface only 
touches the water, one third of the magnifying power is lost ; 
and if the convex surface touches the water, the magnifying 
power is diminished to less than one fourth of what it is in 
air. 
I should add that it makes no difference in respect to the 
magnifying power of the object-glass in air whether the 
submersion tube be on or off, nor, in fact, does it make the 
slightest perceptible difference in any way, whether as regards 
definition or illumination; so that for viewing objects in air 
there is no need to remove the submersion tube at all, which 
may, indeed, be regarded as a protecting cover for the object- 
piece under all circumstances. 
1 The slight loss resulting from submersion may be avoided by closing the 
end of the tube with a plano-convex lens of low power (the plane surface 
downwards) in place of a disc of plain glass. 
