Tennis balls are hollow. A golf ball would be a better analogy.

Placement of the various chemicals within a crystal is very important, but is often very difficult to achieve without extensive equipment. It is difficult to prove as well without very delicate analytical techniques that are able to probe the crystals on the micron or sub micron level.

Besides Iodide, mentioned above, it is possible to include Rhodium, Iridium, Osmium, Lead, Cadmium, Mercury and Copper salts in various locations within a silver halide crystal to control speed and curve shape, as well as latent image keeping and reciprocity. And, just because I list them here does not mean that I advocate their use.

Unless you have a firm grounding in emulsion making chemistry, or you have a fixed formula given to you, you will most likely be groping in the dark. Outside of the patent literature, which is purpously made obscure most of the time, very little of this is published. Mees and James is one of the best textbooks on this subject, but contains little specific information.