What do crystals represent in the context of manual processing?

In the context of manual processing in radiography, crystals represent an important aspect of the film development process. When the film is not sufficiently washed after fixing, residual fixer can remain on the film's surface. This fixer contains unexposed silver halide crystals that have not been removed during the development process.

When these unwashed films dry, the silver halide crystals can appear as white specks or spots, indicating the presence of undeveloped areas. This condition not only affects the quality of the radiographic image but also represents an incomplete processing step, emphasizing the need for thorough washing to ensure all residual chemicals are adequately removed.

In contrast, the other options describe different processing issues: too long in the developer would lead to overdevelopment and dark images, overexposure relates to excessive radiation hitting the film leading to overall dark images, and stuck film in the developer pertains to film handling issues rather than chemical residues on the film. Thus, insufficient washing after fixing is the correct focus regarding the representation of crystals on the film.