118 Transactions of the Royal Microscopical Society. 
in certain cases it may become so dark as to obscure certain cha- 
racters. It is therefore of the very greatest importance to have 
the means of varying the angle of deviation from a direct line, 
which in ordinary microscope apparatus is the most readily effected 
by a diaphragm below the condenser. In the case of very fine 
particles of such substances as pumice, which play an important 
part in the material obtained from great depths in the Atlantic , 
and Pacific oceans, the outline can scarcely be seen with an 
object-glass of even moderate aperture, when they are mounted 
in balsam, if the light use] for illumination be convergent, but 
is seen to very great advantage when illuminated by the divergent 
light obtained by using a concave lens instead of the usual convex 
condenser, that is to say, following out the usual nomenclature, 
when the aperture of the condenser has a negative value, or, so to 
say, is considerably less than nothing. We are, however, limited 
to low powers, by the light being made too feeble for high. 
Object-giasses used. 
The facts just described will make it very obvious that in study- 
ing deposits there is no advantage in having object-glasses of large 
aperture, but a positive disadvantage. When the light transmitted 
is so convergent that the full aperture is utilized, the object becomes 
almost, or quite, invisible from the absence of any dark outline, 
and the focal point of such lenses being so near to their front 
surface it is quite impossible to penetrate sufficiently deep down to 
see the minute fluid cavities in the centre of grains of sand, or to 
reach the fine particles lying on the surface of the glass slip, below 
the thickness of balsam necessitated by the presence of large grains 
of sand. For these researches Messrs. Beck have made for me a 
1th of only 75° of aperture, constructed, however, with all possible 
care, and I find it extremely useful, since I can easily reach all 
parts of the object, and can obtain perfect definition with the power 
of about 600 linear requisite to identify the extremely minute 
fluid and glass cavities. I can also see the form of grains as small 
as sy¢o00 Of an inch in diameter. Such cavities and particles are 
excellent tests for those qualities in object-glasses necessary for the 
present inquiry, since we know what we ought to see. Very few 
objects occur less than those named, and even at that size we seem 
to have arrived at the limit allowed to us in such cases by the pro- 
perties of light itself. If this were not the case, we ought to see 
a bright speck in the centre of the bubbles in the minutest fluid 
cavities; but none can be seen when, calculating from the diameter 
of the bubble, the central bright speck ought to be less than 
evooo Of an inch in diameter. A 3th, with an aperture of 120°, 
