M. C. White on the Microscope. 391 
pressure upon compressible bodies, as air, wood, &c., is to con- 
dense them into a smaller bulk, by which they may be rendered 
heavier than water, and will sink of their own weight. A piece 
of wood cannot float at the bottom of the sea, but a very slight 
extraneous force will bring it to the surface. ; 
Now, how is it with the sounding lead and line? The lead, 
if allowed to descend alone, will fall with a uniform and rapid 
velocity tothe bottom. This velocity will be attained within a 
few feet of the surface, and will be due to the opposing forces of 
gravity and the resistance of the water, which will be balanced, 
when the uniform velocity is reached. But if a line be at- 
tached to the lead, a few hundred feet of the line will offer a 
resistance to the motion nearly equal to the whole weight of the 
lead, and as successive lengths of line are drawn into the water, 
the resistance is constantly increased; so that at 2000 or 3000 
fathoms depth, the weight will be almost entirely suspended in 
the sea by the resistance of the water along the sides of the line. 
Some idea of the resistance which opposes the motion of a 
sounding line, may be formed from the fact that upon 1000 fath- 
oms of a line one-tenth of an inch in diameter, moving with a 
but necessary, before a certain knowledge of those depths can 
be obtained. 
“Arr, XLV.—Application of Polarized Light to the Microscope ; 
by Dr. M. C. WHITE. 
ae: 
For conducting microscopic investigations larized 
light, it is serene to employ two Nicol's prisms, one of which 
is placed below the stage of the microscope, an other just 
light, after it the prism, by means of an achromatic con- 
a But as the Niebol’s rism transmits only the extraor- 
of which it is composed, one-half the light is lost and a power- 
ful light is ae to give adequate illumination. 
dinary ray, produced by double refraction of the Iceland spar 
