546 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
tion. The distance of ten inches has been assumed as the proper interval 
between the objective and eye-piece, that being the average focal length 
of the normal eye, but where to measure from and to is not so easy to 
ascertain. Some opticians estimate from the end of the tube of the 
Microscope, others from the outside of the back lens of the objective, 
others from a point midway between the different lenses of the objective, 
and still others from the front end of the latter. The difference between 
these two extremes is fully two inches, which may cause a difference of 
20 per cent, in the results. Still others choose the point where parallel 
rays sent through the lens from the front would come to a focus, called 
the posterior principal focus, and one or two others the posterior conju- 
gate focus, still higher up the tube, a point where rays meet which 
emanate from another point in front of the objective, at a distance such 
that the size of the object and image are made equal. This is an easily 
established place, but a theoretical consideration of the optical principles 
involved shows that the only proper position from which to measure the 
tube length, is from the posterior principal plane of the objective. In a 
simple leas this is easily ascertained, and in a very thin lens can be 
called the centre of the lens ; but in a complex combination where the 
distance from the front of the front lens to the back of the back lens 
is sometimes two inches, the exact point from which to estimate tube- 
length becomes important. These principal planes in nearly all con- 
verging lenses are situated inside the objective at different distances 
from the centre of the combination, depending on the power of the lens 
and the way in which the corrections are made. In two-system objec- 
tives where the magnifying power is effected nearly equally by both 
systems, the principal planes are near the centre of the systems, while in 
some high power objectives they may cross one another, the posterior 
plane being in front of the anterior. The principal foci anterior and 
posterior of a lens are also two important points to know, and when 
these four data are given they are all that are necessary for a discussion 
of the properties of a lens. 
In the last Journal of the Royal Microscopical Society,* under the 
head of Measurement of Lenses, Prof. S. P. Thompson has given a very 
exhaustive and able article on how to ascertain this point or plane from 
which the 10-in. tube-length is to be measured. The instrument he 
uses is very complex and expensive, but the measurements can be made, 
except for high-power objectives, with a near approximation by any one 
with a little mechanical ingenuity. The principle of the mechanism is 
as follows : — The objective to be tried is placed in a horizontal position, 
and some point on it, either the front end or some point on the side, is 
selected as a zero-point for all measurements, a beam of parallel rays 
is sent through the lens from the back, and the distance from the zero- 
point to the focus F 1 of these rays measured. The lens is reversed, and 
the focus F 2 of the rays issuing at the back is measured from the zero- 
point. Then two small glass micrometers w r ith coarsely ruled lines are 
placed one in front of the other behind the lens, and moved by a screw 
until the image of one micrometer is seen in focus on the other, and the 
lines superimposed. The distance of these micrometers from the zero- 
point is measured. We have now all the data for calculating the 
* Feb 1892, pp. 109-135. 
