DIVISIONS OF HYPERMETROPIA  
By C. H. BROWN, M. D. 1921

Original hypermetropia may be divided into manifest and latent, and in order to ascertain the total hypermetropia it is necessary to add the manifest to the latent.

In hypermetropia of not too high a degree it is usually found that the distant vision is quite up to the standard, and the sight is apparently that of an emmetropic eye. This is accomplished by means of the accommodation, which increases the convexity of the crystalline lens and adds to the refractive power of the eye, and thus bends parallel rays so as to advance their focus from behind the retina on to this structure. It is the same effect that is produced by a convex lens placed in front of the eye ; and the amount of accommodation required, which can be expressed by a certain number of convex lens, will represent the degree of hypermetropia present.

The division of hypermetropia into manifest and latent depends on the action of the accommodation ; manifest hypermetropia is possible only with a suspended accommodation, while the latent form is that which is concealed by the contraction of the ciliary muscle. Hence it follows that the more passive the accommodation, the greater the manifest hypermetropia ; and the more active the accommodation, the greater the latent hypermetropia.

The manifest hypermetropia is usually apparent without a mydriatic, and is measured by the strongest convex glass that will be accepted for distant vision. The latent defect can be made manifest, or can be detected only by the use of a mydriatic. As the person advances in life and the power of accommodation weakens, in the same proportion the latent defect decreases and passes over into manifest, until finally there remains no more latent trouble, but it has all become manifest.

The manifest hypermetropia is, for the purposes of convenience, written Hm, the latent hypermetropia HI, and the total hypermetropia Ht.

Perhaps this subject can be made more clear by the exemplification of a case of hypermetropia tested at a distance of twenty feet with Snellen's test types. Possibly the eye can distinguish only the larger letters, and the vision would be recorded as follows: V = 20/100.  If not there is placed before the eye a convex lens of 2 D., the vision is raised to normal, and V = 20/20. In this case the accommodation is supposed to be at rest, and the total hypermetropia (Ht) is 2 D.

If, however, the eye would call into action a portion of its accommodative power, the hypermetropia would be corrected thereby and the vision raised to 20/20 without the employment of a convex lens, and this is usually the state of affairs as it is found in young hypermetropes. If now the eyes are tested with convex lenses, the vision remains the same, so that we find V 20/20 either with or without convex lenses. This illustrates manifest hypermetropia, and the strongest convex lens through which vision still remains 20/20 would represent the degree of manifest hypermetropia (Hm).

The record of this case would read V = 20/20, Hm. = + i D. That is to say, vision is normal or 20/20, and remains as good when a convex lens of i D. is placed before the eye. Now in this case we presume that a certain amount of the defect is latent or concealed by the action of the accommodation, because the patient is unable to completely relax it.

In this imaginary case we have a total hypermetropia of 2 D., and a manifest hypermetropia of i D., and therefore the difference between the two would indicate a latent hypermetropia of i D.

The latent hypermetropia can seldom be revealed except by the use of atropine, and we would re nark in passing that this is not always necessary for the following reasons: we can scarcely ever *i ve a glass to do more than correct the manifest hypermetropia, which can be measured without the use of the drug; why then should it be considered essential to determine the latent defect, which after all will not bear correction.

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