In many of our instantaneous perceptions these two stages are indistinguishable to consciousness.

Thus, in most cases, the recognition of size, distance, etc., takes place so rapidly that it is impossible to detect the two phases here separated.

But in the classification of an object, or the identification of an individual thing, there is often an appreciable interval between the first reception of the impression and the final stage of complete recognition.

And here it is easy to distinguish the two stages of preperception and perception.

The interpretative image is slowly built up by the operation of suggestion, at the close of which the impression is suddenly illumined as by a flash of light, and takes a definite, precise shape.
Now, it is to be noted that the process of preperception will be greatly aided by any circumstance that facilitates the construction of the particular interpretative image required.