Full question: How did they ever find the chemical that makes your pupils dilate? Was it a beautifying technique in Victorian times (or in the past 200 years)?
Donald Mutti, a professor at Ohio State University's College of Optometry, eyes an answer to this query.
Our pupils naturally dilate in darkness and constrict in bright lights through the actions of two opposing muscles in the iris, the iris dilator and sphincter. It is these iris muscles that actually do the work; the pupil is just a hole in the iris, which is the "curtain" of the eye that contains our eye color and controls the amount of light traveling toward the retina.
The dilator muscle is arranged in a radial pattern. Its contraction pulls the iris outward, bunching it up like a curtain drawn open. The iris sphincter is arranged in a circular pattern, similar to a purse string. Its constriction pulls the iris inward and pulls it flat, like a curtain drawn closed.
These iris muscles are under the control of the autonomic nervous system, which deals with involuntary reflex actions. Sympathetic output, which is associated with arousal, stimulates the dilator muscle to constrict, opening our pupils during a "fight or flight" situation. Parasympathetic output, more closely associated with calming mechanisms, stimulates the iris sphincter to constrict, shrinking our pupils.
Dilating drops work by blocking parasympathetic receptors in the iris sphincter, allowing the iris dilator to act unopposed and enlarge the pupil. (The pupil dilates mostly because it can't constrict.) This parasympathetic input also controls accommodation, or changing focus of the eye for near objects. Dilating drops block that input too, making near objects appear blurry when the drops are active.
These drops are anticholinergic agents, blocking the effects of acetylcholine, the neurotransmitter released by parasympathetic nerve cells. Modern dilating drops are just synthetic cousins of their ancestor atropine, an extract of Atropa belladonna (also known as deadly nightshade). Atropine is a notorious poison, responsible for the famous quartet of signs that indicate someone has ingested the toxin: "red as a beet, dry as a bone, blind as a bat and mad as a hatter." Belladonna, translating to "pretty lady," refers either to the beauty of the flower, the beauty of the rival poisoned by atropine or its use as a cosmetic, according to the Oxford English Dictionary.
One would only have to rub an eye after preparing this extract to discover its pupil-dilating effects. Apparently this was exploited, particularly in Italy, by women who wanted to adopt a doe-eyed appearance by creating large pupils. The sight of one's beloved with pupils enlarged had the desired effect of communicating arousal.
After all, the sympathetic nervous system is more than just fighting or fleeing.