Here we have pictured white light entering the prism and what can be seen behind the prism as light widens to form a continuous colored spectrum (screen is placed in the plane of the illustration paper). Immediately after light exists the prism we see a beam of white light with colors only at its borders. The colored borders get wider and the white area in between narrower as the distance from the prism increases. Only at a certain distance from the prism the spectrum comes to resemble the complete continuous spectrum illustrated in physics text books. 

The partial 'unfinished' spectrum, that included white light is known as boundary, or edge spectrum. Although physics books avoid mentioning it in this context (it is crucial in physics to determine the complete spectrum before the presentation of the edge spectrum), it is a familiar phenomenon for the physicists and its explanation is considered uncomplicated and simple. In the circles underneath the spectrum is illustrated what can be seen on the screens set at right angles (blue vertical lines) to the gradually widening spectrum.