instrument, it is imperative to carry out another different test: determination of 
the resolving power of the instrument. This test should include the complete pro- 
cedure from original negative to the image as observed when plotting the map. 
The simplest method is obtained by producing a number of photographic copies 
of the same test tables as are used for the determination of the resolving power of 
photographic objectives. It is advisable to copy. these test tables at at least 15 
points of the same diapositive plate. A provisional solution would be to stick 15 
small diapositives of this plate in the proper position directly on to a blackened 
diapositive. Test tables with a slight contrast should be used for this purpose. 
Such a plate may be placed in the instrument to be tested and the number of 
lines per millimetre still visible separately, may be counted. For instruments neces- 
sitating a reduction of the original image, the plate with the 15 small images 
should be reduced accordingly and the result observed in accordance with the 
normal procedure on the instrument in question. Also for multiplex-like instru- 
ments we obtain by this procedure an objective possibility of comparison for the 
general resolving power of the whole plotting procedure. 
It is clear that besides the measurement of grid plates, where the resolving 
power plays only a minor role, the aforesaid indication of this resolving power is 
necessary for the total evaluation of a certain plotting machine. 
It should be considered, however, that also for the evaluation of the resolving 
power — just as for the accuracy — the work to be carried out by the instrument 
plays an important part. For the operator who has nothing else to do than to 
trace contour lines in hilly jungle terrain, the resolving power is of less importance 
than for the photogrammetrist who has to produce large-scale maps of highly cul- 
tivated areas and who has to plot very accurately frontier stones of 1010 cm 
which are hardly visible in his original photograph. 
The qualities of the instrument, including the accuracy as obtained from grid 
measurements as well as the resolving power of the total procedure should never 
be considered in themselves, but always in connection with the problems to be 
solved. This is xo simple matter which can be solved, by means of a few figures 
as indicated in Mr. Pennington’s proposal, by organizations who wish to buy 
photogrammetric instruments, but who have not the necessary experience.