On the Evolution of the Fine Adjustment. Bj E. M. Nelson. 373 
the motion of the nose-piece follows the apparent motion of the 
screw, as is the case with the Continental Microscope, whereas in 
Powell’s plan the motion is reversed. The speed of Watson’s fine 
adjustment is very slow, being 1/359 in. for one revolution. This is 
three-and-a-half times slower than that of the Continental Microscope ; 
but it will not be found to be at all too slow, because any lens pos- 
sessing a fair optical index becomes excessively sensitive to focal 
adjustment when a 3/4 illuminating cone is used. The reason why 
biologists are content with the quick-moving Continental fine adjust- 
ment is because they only employ small cones of illumination, under 
which conditions lenses are far less sensitive to focal adjustment. 
The table of apertures and resolving powers, &c., for many years 
printed by this Society on a fly-leaf of the Journal, has unfortunately 
caused a great deal of misunderstanding upon this and other points 
with regard to the action of lenses. It is tacitly assumed that a lens 
with a given aperture has both definite penetrating and ilium mating 
powers ; this, however, is not the fact, because the table assumes a 
full illuminating cone in each case. Now, if a full illuminating cone 
is used with any lens, except it be of the lowest power, such as a 
1-in., 2-in., or a planar, the object will become invisible, or nearly so; 
therefore this list of penetrating and illuminating powers is merely a 
theoretical abstraction, because the conditions imposed prevent their 
practical employment. It is never realised that a 1/6 of N.A. 0 * 8, as 
used by a biologist with a 1/4 cone, is less sensitive to focal adjust- 
ment than a 1/2 of 0 * 5 with a 3/4 cone ; the magnifying powers being 
the same in both cases. It is only those who use their lenses with 
full battering charges that can appreciate the luxury of a slow fine 
adjustment ; those, on the other hand, who degrade their objectives 
by employing narrow cones, and who use an oil-immersion 1/12 with 
a 1/1 cone to view objects that could be better seen with a dry 1/4 
and a 3/4 cone, or a dry 1/6 of 0’9 N.A., to examine an object that 
could be better seen with a 2/3 of 0*3 N.A., may be able to jog along 
with a quick fine adjustment, but we cannot say that their example 
is one to be imitated. 
In conclusion, let me say that I have not thought it advisable to 
insert any figures in this paper to illustrate the fine adjustments on 
the sixty Microscopes mentioned ; it was impossible to publish them 
all, and a selection would not be of much use. A bibliography is 
therefore appended showing where these illustrations may be found 
in a few well-known books easily accessible to all. 
Bibliography. 
The following references will be found in Quekett on the Micro- 
scope, 2nd edition, 1852: — (3) p. 8, fig. 4. (10) p. 11, fig. 8. (12) 
p. 12, fig. 9. (14) p. 15, fig. 11. (32) p. 105, fig. 52. (33) p. 102, 
fig. 50. (34) p. 104, fig. 51. (36) p. 355, fig. 235. (37) p. 107, fig. 53. 
(40) p. 61, fig. 38. (43) pi. 4. (46) p. 75, fig. 43. (49) pi. 2. (50) pi. 1. 
1899 2 c 
