502 REPORT—1890. 
There would be no difficulty in constructing satisfactory instruments 
of this variable-angle class were it not for the fact that, the base being 
necessarily very limited in length compared with the ranges to be 
determined, the angles to be dealt with are exceedingly small, and 
consequently the slightest relative movement of the mirrors puts the 
instrument out of adjustment. Thus, let us assume that in the 
instruments themselves, independently of errors in observation and 
alteration in the length of the cord, there is not to be an error of more 
than 1 per cent. at 1,000 yards,! and that the base is 25 yards. This 
means that the angles must be set out correctly to about 50” ; and since 
by reflection any angular shift of a mirror is doubled, a shift of any 
one of the four mirrors, or of these collectively, of more than 25’ is 
inadmissible. Moreover, the mechanism operating the movable mirror 
and scale must work correctly to 25’. This necessitates the use of 
micrometric devices of great delicacy. Such instruments are, therefore, 
not only liable to derangement by slight displacement of the mirrors, but 
micrometric arrangements, however perfect at first, are subject to wear 
and to rusting, especially under the conditions of actual service in the 
field, where they must necessarily be exposed to the damaging effects of 
rain and dust. 
Colonel Weldon’s instrument has a great advantage over instruments 
of the variable-angle class as hitherto made, in that it is incapable of 
derangement; but it has what appears to us to be the almost equally 
great disadvantage, that it is nothing like so facile in operation, especially 
for taking the range of an enemy in motion. 
Our object in designing the range-finders we are about to describe 
has been to overcome the difficulties to which we have referred, and to 
combine the invaluable feature of non-liability to derangement with the 
facility of operation which is the characteristic of instruments based on the 
constant-base system. We believe that these three instruments are unique 
among constant-base range-finders in being quite incapable of optical 
derangement, whatever the treatment to which they may be subjected. 
In each of the three range-finders at present to be dealt with, we use 
two instruments connected by a base-cord in the manner above described. 
The cord should, of course, be as inextensible as possible, and should not 
be subject to alterations in length through dampness. Fishing-line can 
readily be obtained very suitable for the purpose. Small variations in the 
length of the cord cause, however, no appreciable error—e.g. a variation 
of length of ] inch in 25 yards will only affect the indication of the range 
by 1 part in 900. 
The first of the three instruments is operated in the manner described 
with reference to other constant-base instruments, and illustrated in fig. 4. 
The instrument carried by the assistant observer is an optical square, but 
instead of heing formed in the usual way of a pair of mirrors fixed at an 
angle of 45° to each other (fig. 3), it consists of a doubly-reflecting prism 
of a pentagonal form (fig. 5). The faces AB and BC are at right angles 
to each other, and the faces AB and cD, which are silvered, are inclined 
at 45° to each other. If the angles between these pairs of faces be exactly 
90° and 45° respectively, then the prism forms a true optical square in- 
capable of derangement. The course of a ray of light through the prism 
is represented in the figure. These prisms possess a great advantage 
1 This appears to us to be a large error to assume as allowable. 
