doctrine, the pioneer inventor could gather little fruit from his
patent, for the patent could be avoided, perhaps, by the mere
substitution of a wedge for the screw or lever called for by the
claim. The court, having ascertained from the prior art that the
inventor is entitled to invoke the doctrine of equivalents, will
proceed to ascertain if the substituted elements are real equivalents.
A given omitted element will be considered in connection with its
substitute element, and if the substitute element is found to be an
element acting in substantially the same manner for the production of
substantially the same individual result, and if it be found that the
prior art has recognized the equivalency of the two individual
elements, then the court will say that the substituted element is a
mechanical equivalent of the omitted element, and that the two
combinations are substantially the same. This reasoning must be
applied to each of the omitted elements for which substitutes have
been furnished. In this way justice can be done to the pioneer
inventor. But the courts, in exercising liberality, cannot do violence
to the language of the claim. The infringer will not escape by merely
substituting equivalents for recited elements, but he will escape if
he omits a recited element and supplies no substitute, for the courts
will not read out of a claim an element which the patentee has
deliberately put into the claim, and a combination of a less number of
elements than that recited in the claim is not the combination called
for by the claim.

It is seldom that the exemplifying device of the pioneer inventor is a
perfect one. Later developments and improvements by the original
patentee, or by others, must be depended on to bring about perfection
of structure. Those who improve the structure are as much entitled to
patents upon their specific improvements in the device as was the
original inventor entitled to his patent for the fundamental device.
These improvers are secondary inventors, and are not entitled to
invoke the doctrine of mechanical equivalents. The secondary inventor
did not bring about a new result, but his patent was for new means for
producing the old result. His patent is for this improvement in means,
and his claim will be closely scrutinized in court, and he will be
held to it, subject only to formal variations in structure. The
justice of thus restricting the claim of the secondary inventor must
be obvious, in