De praecipius visus symptomatis eorumque causis Physica & Medica contemplatio

LXXIIII.
To the diminished action of seeing, several types are referred, the first of which is amblyōpia dull/weak vision, a blunt and more sluggish sight proceeding from an internal cause, which Galen attributed to either the spirit or the instruments of the sensorium.

LXXV.
A spirit that is abundant, thin, and bright renders the sharpness of the eyes most subtle in looking at all objects; so, the same spirit, if thin and serene but deficient in quantity, sees things nearby, but does not attain things precisely that are remote. But if it is thick, turbid, and abundant, it is blind to things nearby which are before the feet, but is sharpened in more remote things so that it may comprehend the form. If it is sparse, it brings on sluggishness, recognizing neither nearby nor remote things.

LXXVI.
Galen described the affections of the optic spirit with this distinction, in which, however, some limitation must be applied, so that we may not, as is commonly done, be carelessly rolled back into the absurd hypothesis of the author regarding the emission of spirits.

LXXVII.
Physicians established a certain thin, bright essence, composed of water and air, for the primary organ of vision, hōs atmōdē as vaporous in respect to another boiling over from the heart, which it is permissible to compare to an anathymiasin exhalation. This [spirit], the more fervent it is, the more thin and wandering it is in perennial mobility, and thus plainly inept for sensation; the other, more temperate and corpulent, yet participating in the most pure matter, has received a different condition, and that pathētikēn kai drastikēn passive and active at the same time.