Excerpt from Optical Defects in School Children: An Address Before the Massachusetts Teachers Association at Its Annual Meeting, October 17, 1868

I would show, also, that the threatened misfortunes must be averted through the intelligence and vigilance of teachers, since, in many instances, the pupil himself will make no complaint, but will even be unconscious of the dangers he is incurring till the mischief is beyond repair.

Within a very few years, great advances have been made in a knowledge of the optical conditions necessary to perfect vision, and of the causes which disturb the normal exercise of this function. Till recently, much uncertainty existed as to whether the imperfect sight, or inability to use the eyes, frequently complained of, depended on optical defects or on organic structural changes. But the invention of the ophthalmoscope by means of which we are able to illuminate and explore the interior of the eye, has made it possible at once to detect the latter, besides aiding in determining the presence of the former. It shows, moreover, in certain cases, the co-existence of both these morbid conditions, and their mutual dependence.

Not to trespass upon your patience, I leave out of question, in these remarks, such defects of sight as are due only to structural changes, - though many of these so far lessen the visual power as to allow of only moderate and cautious use of the eyes, - and shall ask your attention to some of the more common disturbances of the refractive and accommodative powers of the eye.

The normal eye, when looking at distant objects, (such as are at or beyond eighteen or twenty feet from it, the rays from which are considered as virtually parallel, ) may be regarded as in a state of rest, and as an organ of refraction merely. Such parallel rays are without effort brought to a focus upon the retina, which forms a distinct image of the object looked at, and conveys the impression to the brain through the medium of the optic nerve. In order to the full perception of this mental impression, both eyes must receive an image upon corresponding portions of their retinæ, - any want of harmony in the direction of the axes of the two eyes, or any loss of perceptive power in either, at once rendering vision in a measure defective or confused.

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