bilities suffer from a certain degree of intolerance of light. To avoid the inconvenience arising from ordinary daylight, it is a not uncommon practice to employ tinted glasses. It is even the practice of some oculists to prescribe such. The practice is not one to be commended. If the eyes do not tolerate the light, the reason for the intolerence should be learned and removed. Proper attention to the refractive or muscular states will, in the great majority of instances, afford complete relief.

In case of disease of the eye, or in facing extreme light, colored protectors may be of temporary advantage.

The material of which lenses should be made, and the manner in which they should be adapted to the face, are subjects worthy of consideration.

Many people suppose that "pebbles" or lenses made of rock-crystal are much better than those made of glass. This is a popular error. The crystal has only the advantage of greater hardness, while it has the disadvantage of greater expense, and is very often less perfect optically than the glass lens.

Glasses should be so adjusted to the face as to bring them in proper relation to the eyes. Formerly nearly all eye-glasses were so made as to hang downward upon the face, greatly interfering with the symmetry of the facial lines, and forcing the eyes to look through the borders of the glasses.

Recently much improvement has been made in this respect. Such glasses should be made to permit the light to pass directly through the optical center of the

glass to the pupil. The borders should correspond with the lines of the brows. The glasses should be large, and the frames should not be conspicuous. Under these circumstances the natural expression of the face is not interfered with, and the glasses are much less conspicuous than when the facial lines are broken up.

There is a general prejudice against the early employment of glasses for presbyopia. It is thought that the eye should be forced to perform its function as long as possible without artificial assistance. This, if the condition were one of temporary failure of muscular tone, might be logical. In the actual state of the eyes such a prejudice is unwise. The eye in presbyopia is required to exert an amount of force which is entirely inconsistent with the well-being of the eye itself or of its possessor. If one persists in forcing the eyes to do close work without glasses after presbyopia has commenced, the muscular power fails, and presbyopia increases more rapidly than if proper relief is given at the right time.

AFFECTIONS OF THE OCULAR MUSCLES IN WHICH BINOCULAR VISION MAY BE MAINTAINED.

IN the study of the relations of ocular conditions to disturbances of the nervous system, the affections of the ocular muscles occupy a position of paramount importance.

The complicated system of muscles which co-operate in adjusting the two eyes in such a manner as to obtain binocular vision under a multitude of circum

stances, affords a subject of research attended by difficulties but rich in interest.

In the act of binocular vision-that is, of vision in which the object seen by the two eyes makes but a single mental impression-the principal optic axes are in such exact relation to each other that a straight line drawn from the object through the pupil falls upon the yellow spot of the retina, the central point of vision of each eye, and at the same time each eye must be accurately adjusted in respect to its focus for the distance from it to the object seen.

With every new adjustment of the eyes their relations must be so precisely maintained as to permit the line from the point seen to fall upon this minute portion of the retina of each eye.

Such ever-changing and extremely nice associated actions are demanded in no other part of the organism. The movements of the extremities, no matter how precise or how delicate, make no such constant demand for minute precision; and from no class of muscles, other than those that direct the eyes and regulate the accommodation, is the maintenance of perfect exactitude of service so constantly required.

That this exacting service should, when difficulties in its performance are encountered, make excessive demands upon the stock of nervous energy of the individual, or result in perplexities or irritations, is not surprising.

Affections of the ocular muscles may be divided into those which result from physiological peculiarities and those which result from pathological conditions.

In the first of these groups, the muscles, while manifesting no indications of disease, do not act in such harmony as to permit the most ready and easy combinations of action. This group is divided into two classes:

1. Those which permit of habitual binocular vision. 2. Those in which a blending of the images of the two eyes is so difficult as to be, in most instances, impossible. The conditions of this class are known under the general term strabismus.

The first of these classes has for a long time been known under the name of insufficiencies of the ocular muscles.

For reasons which have been fully discussed elsewhere,* this term is regarded as indequate and often misleading. It has been shown that for some of these conditions no distinctive terms exist, and that to others the term insufficiency is improperly applied. Terms of more exact meaning are therefore required.

Accordingly, the system of terms relating to the conditions, which was suggested in the works referred to, will be employed here.

In this class of muscular faults, binocular vision is maintained by the expenditure of a greater amount of force than is required when the ocular muscles are in a state of perfect equilibrium. The visual lines are habitually held in such relations as to extend from the point of fixation to the yellow spot of the retina, but

*" Archives d'Ophthalmologie," Paris, November, 1886; "New York Medical Journal," December 4, 1886; "Archives of Ophthalmology," New York, June, 1886.

only by persistent and special effort. The tendency is for the visual lines to part, for one of them to continue to unite the fixed point and the macula or yellow spot and for the other to fall upon some other part of the retina. Such tendencies are grouped under the generic name HETEREPHORIA (Tepos, different; pópos, a tending).

This term includes the conditions which have been known as insufficiencies of the ocular muscles.

Some of the most distinguished contributors to the science of affections of the eyes have given considerable attention to this subject, yet it has received vastly less consideration than its importance has demanded. To Graefe we are greatly indebted for important researches in this department; and Horner, Nagel, Landolt, and many others, have made valuable additions to the subject. The writings of Graefe were those of a pioneer and were not exhaustive. Others, however, have been content in great measure to accept the results of Graefe's genius as in the main conclusive. The discussion of "insufficiencies" has been mainly, it may be said almost exclusively, confined to a single anomaly, and that not the one of greatest importance.

When the eyes are directed to a distant object situated directly in front of the observer and at a distance of from fifteen to twenty feet, the visual lines are practically parallel, and in this position there should be the minimum of nervous energy directed to the muscles of the eyes. If this is the case, the ocular muscles are said to be in a state of equilibrium and in all other adjustments the changes of relations required are made