1831.] 
Notices of European Science. 
31 
XI. — Notices of European Science . 
1. — Captain Kater’s Collimator. 
An instrument, under this title, has been invented by Captain Kater, which pro- 
mises to be of great use to the astronomical observer. A detailed description, and 
account of its performance, will be found in the Philosophical Transactions. 
The following short notice may give our readers some idea of it. 
In observing the altitude of a celestial body, we require to know the place on our 
circle, either of the zenith, or the horizon. The former is given by the plummet, the 
latter by the spirit-level. The plummet is liable to many objections, such as agitation 
from any slight motion of the air ; and parallax, which occasions an error when 
judging of its coincidence with the dot on the circle. If the distance between the 
wire and the plane of the circle be again diminished till it touch, there is no longer 
that perfect freedom required, to give the wire the vertical direction. These objec- 
tions have occasioned the level to be very generally preferred ; but even the latter is 
by no means free from objections. Thus it is a very difficult thing to grind a level so 
true, that whatever the expansion and contraction of the bubble, its indications will 
be equally correct. And any inequality of temperature, however correct it other- 
wise may be, must inevitably give rise to error. The position of the observer occa- 
sioning an unequal distribution of heat, will often be sufficient to disturb a delicate 
level, as every observer, who has used them, knows, and a fortiori , the application of 
the hand to reverse them. That both plumb line and level are liable to serious 
objections in practice, will be sufficiently evident to those who will advert to the 
practice, we believe first introduced by the present Astronomer Royal, of referring all 
h.s observations for the declinations of the fixed stars to the polar point, or place of 
the pole ; which he determines independently of either plumb-line or level, by ob- 
serving the place of circumpolar stars, both below and above the pole. The height 
of the pole, again, is determined, by observing the direct place of the star, and its 
reflected place, as seen in mercury or water. 
The latter practice, when it can be had recourse to, is, perhaps, as good as can be de- 
sired, and certainly decidedly superior to either plumb line or level. The objection 
to it is, that the time lost in changing the direction of the telescope, occasions a 
change of altitude in the star ; and that, requiring, as it does, the most perfect stillness 
for its successful practice, that stillness must occur at the time the observation is 
required, or it cannot be taken advantage of. This objection is obviated by Captain 
Kater’s instrument, which being also founded on the same principle as the other, i. e. 
the perfect horizontally of the surface of a fluid at rest, has all its advantages, with- 
out its defects. By means of the collimator the horizontal point can be fixed at 
any time, without any reference, whatever, to the place of the stars. 
To understand, clearly, the nature and value of the collimator, we shall begin by 
stating a fact, which may be new to some of our readers, but which they may easily 
verify, if they please. If two telescopes, fitted up with cross wires, be adjusted to the 
distinct vision of distant objects, and placed with their object ends adjacent, (no 
matter at what distance,) and in the same straight line ; then, if we look through 
one of the telescopes, we shall see a magnified image of the cross wires belonging 
to the other; and we may, if we please, make them coincide with the wires of the teles- 
cope we are looking through, to any degree of nicety ; (the reason of this will be ob- 
vious to any one acquainted with the elements of optics.) This is one of the facts 
on which the use of the collimator depends, the other is the following. A body, 
floating on the surface of a fluid, will, in similar circumstances, have the same position. 
By similar circumstances is meant, being placed in the same direction, and having no 
more sources of disturbance in the one case than in the other. 
Now let us suppose a vessel, of a square form, filled with mercury, and having a float 
of the same size (nearly) and shape. To this float let a small telescope, with cross 
wires, and adjusted to distinct vision, be firmly fixed ; we shall have Captain Kater’s 
collimator. For its use : Let us suppose the whole apparatus placed to the north of our 
circle, and at such a convenient height, that when the telescope of the circle is pointed 
(by estimation) horizontally, the cross wires of the collimator will be visible ; 
let the wires of the telescope circle be brought accurately upon those of the collimator, 
so as to coincide most perfectly ; this is, of course, easily done by the tangent or slow 
motion screw of the circle ; the stand of the collimator being elevated or depressed, 
as occasion may require. Let a reading be now made, and the collimator being 
tranferred to the southern side of the circle, the same operation is to be repeated. 
The mean of the two readings will, of course, give the inclination of the collimator, 
supposing it to be not perfectly horizontal ; and the difference of either reading from 
the mean, will give the error of zero, which being applied -f- or- — as the case may 
require, the place of the horizontal point is thus obtained. 
