FO TO GR A MME TRIE
R. ROELOFS,
A method of measuring the flatness of photographic
glassplates, the locating back of aerial cameras, etc.
Professor at the Delft University:
A method is described which requires a collimator, a transit instrument (or a
theodolite) provided with an eyepiece micrometer and a small triangular prism.
It may be of use for laboratories which do not have a reference surface plate.
I. Description of the set up.
The surface 6" to be measured is mounted horizontally on a stable
support between the collimator C and the transit instrument T (fig. i).
The collimator is levelled and its horizontal and vertical wires are
illuminated askantly by two small lamps L. The prism P on whose
Fig. i.
lower face two small half balls are luted, is placed on top of the surface
to be examined, in the line of sight of the collimator. A screen Sc is
attached to the prism to prevent superfluous light from reaching the
transit instrument. In order to keep the prism and screen upright and to
diminish its pressure on the surface, it is suspended by means of two
