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nylon wires from a simple balance B with an adjustable counter weight
IV. The nylon wires are fastened to the sides of the screen, right
above the horizontal line through the centre of gravity to bring prism
and screen into almost labile equilibrium so that it may easily contact
the surface with the half balls. The position of the prism can be varied
by shifting the balance along a bridge Br which is parallel to the colli
mator's line of sight.
The pencil of light reflected by the wires emerges from the collimator,
passes through the prism and enters the objective of the transit instru
ment to form an image in its focal plane (fig. 2). The image is seen
as a bright cross against a dark background. It is evident that the
elevation of the pencil of light which emerges from the prism and
hence the position of the images of the horizontal wire depends upon
the inclination of the prism. A variation in this inclination will displace
the image twice the amount, reckoned in angular measure. This dis
placement can be measured with the eyepiece micrometer M.
2. Procedure.
After the balance has been brought into perfect equilibrium two
weigths of 1 gram each are put on the balance arm above the prism.
Thus the pressure of the prism on the surface is two gram only. By
shifting the balance the prism is placed in successive positions in the line
of sight of the collimator, with intervals equal to the distance between
the half balls (25 mm in our case) and each time the position of the
horizontal wire's image is measured with the micrometer. Thus the
differences in gradient of adjacent profile segments of the surface
are determined. The elevations of the terminal points of the segments
with respect to some reference level can easily be computed from these
differences. Several profiles are measured in this way.
3. Examples.
Fig. 3 is given as an example. It shows twelve profiles of a Kodak
18 X 18 cm2 glass plate. The profiles 1-5 are parallel to one side of
the plate (interval 35 mm), 6-10 are parallel to the other side, while 11
and 12 are profiles along the diagonals. The line connecting each pro
file's terminal points has been used as a reference level. The mean error
of an elevation read from these profiles is 0,0027 mm.
Since the profiles have some points in common it is possible to refer
them to one common level by adjustment and to employ the elevations
thus obtained for drawing contourlines representing the actual con-
Transit instrument
Pencil of light
Surface
Collimator
Micrometer
Fig. 2.
