legible. The counters were much the same as before. The restrictions on strings
of dots or crosses and on minimum text length remained.
(d) The basic weakness of Robinson.
The disadvantages of Heath Robinson listed above were later overcome, but
there is one which is inherent in the Robinson principle, namely, that a pattern
cannot be "extended", in particular, in psi-setting, because the psi pattern could
not be extended, it was necessary to "contract" the de-chi, i.e. letters opposite
a total motor dot were omitted. This wasted evidence, but was quite feasible with
no limitation or X2oneback limitation.
A related functional disadvantage is that stepping is necessarily uniform,
so that to set wheels arbitrarily is extremely laborious, moreover when a wheel
which has been stepping, is to remain at a fixed setting, its tape must be
replaced by one of different length.
(e) Colossus 1
The flexibility of Heath Robinson for experimental purposes made it easy to
discover the essential requirements of a Tunny-breaking machine. As a result, Colossus
1, the original experimental model, really lacked surprisingly little for a first
model. The choice of runs, though more extensive than on Robinson, was less
extensive than Heath Robinson had shown to be desirable: it was biased towards
runs of the form i+j = .: these could be done by switching except in the fifth
counter. Most other runs required plugging, though there was a single
set of five dot and cross switches for "all counters". There were five counters,
two pairs of which could be used independently for double testing on X1, but for
this it was necessary to set up the same wheel twice with a stagger of one.
Operation was not very simple because of the lack of symmetry, accentuated
by changes introduced without correcting the "signwriting" on the machine. There
was no spanning and only a single bedstead.
(r) Colossus 2 and later.
Experience gained from the development of Colossus 1 added to that from
Heath Robinson, made possible Colossus 2, the prototype of all later Colossi, in a
form which needed very little modification.
Colossus 2 possessed from the first, quintuple testing, a generous switch
panel (including not-not), a versatile plug-panel, spanning, a double bedstead,
and a greatly increased simplicity of operation.
Spanning was introduced originally for P5 limitation, but was soon found
indispensable for all setting.
The chief modifications introduced later were the rectangling gadgets,
devices to reduce the effect of doubtful cipher letters, and devices to make