Health - What is the physical cause of glassy eyes?

[fidens]

SWIM's pupils never dilate on cocaine, at least not to his knowledge. That might, however, be due to the fact that he always does fairly low amounts which contain no active chemicals other than cocaine.

What he does regularly notice is that glassy effect which takes over the eyes. Even on the lowest doses it's fairly noticeable.

What swim is wondering is, what physically causes the glassy eye look? It's as if a transparent film covers the surface of the eye which reflects more light. Swim does not think it's an issue of blinking, but he could be wrong.

Does anyone know the actual reason?