Following the home-made remedies of salty water, the evolution of dry eye therapy continued with commercial preparations of physiological saline becoming available, this time with specific salts and minerals (electrolytes) to make it more resemble the human tears in respect of its acid/alkaline balance (pH) and their concentrations (osmolarity).
This made them at least a little more compatible with human tears and the eye, and nearly all contained preservatives to reduce the risk of drop contamination while in the bottle. Early additions to these formulations were components known as demulcents, which form a soothing, protective film on a mucous membrane surface.
Examples would be hydroxypropyl methylcellulose (HPMC) and carboxymethylcellulose (CMC). These agents help the drop to stay in contact with the existing tears and ocular surface for longer.