It has already been said that a hypothesis should be adequate. Adequacy is one essential requirement for indirect verification.
A hypothesis is taken to be adequate when it is found to be the only possible one to| explain the fact.
Ptolemic hypothesis was considered to be inadequate because it would not explain the phenomenon of aberration of light. On the other hand the Copernican hypothesis could explain the facts in a better way through the supposition that the sun is a the centre and the planets move around it. For this reason the Copernican hypothesis was considered to be adequate one.
Thus for verification adequate hypotheses are helpful because they can explain the complex phenomena from various angles.
In course of verification of a number of hypotheses in respect of some particular event, there may be the need to select one and reject others on the basis of a special ground That special ground, which becomes a crucial factor to show right direction to select the correct hypothesis, is known as crucial instance.
In case of a cross road, selection of the right direction is done referring to some vital mark or finger post. Bacon considers a crucial instance to serve like a finger post.
A crucial instance, Jevon holds, not only helps in selecting the right one, it also helps in rejecting the irrelevant hypotheses. Crucial instances may be found by simple observations or experiments.
The theory of ‘aberration of light’ was found to be the crucial instance to justify, the Copernican hypothesis in respect of the movement of planets.
Similarly, a crucial factor in case of experiment decides the relevance of reaching at a definite conclusion. Such experiment is called “experimentum crusis”.
Scientist Raleigh’s claim that there is another gas in the air other than nitrogen and oxygen was accepted on the significant experiment where argon could be isolated form air. This is considered to be an example of experimentum crusis.