in·still also in·stil (ĭn-stĭl′)
tr.v. in·stilled, in·still·ing, in·stills also in·stils
1. To introduce by gradual, persistent efforts; implant: "Morality ... may be instilled into their minds" (Thomas Jefferson).
2. To pour in (medicine, for example) drop by drop.http://www.thefreedictionary.com/instilInstil means to add dropwise - they are called "eye
drops" for a reason, because they are added dropwise ;@).
Prescriptions and instructions for use include phrases such as, "instil 3 drops in each eye."
As the dosage is measured in drops and instil means to add drop-wise they go together like salt 'n' pepper.
Yes, it is the register used by ophthalmogists.