Page:Handbook of Ophthalmology (3rd edition).djvu/55

MYOPIA, TREATMENT. 49 uous use of neutralizing glasses. Qne often sees myopes who wear their glasses continuously, and with only good effects.

If any one of these four conditions be wanting, we have a contra-indication for the continuous use of neutralizing glasses. A more exact explanation of these conditions belongs with the consideration of tlie above-named contra-indications. The cir- cumstances which make the use of neutralizing concave lenses unadvisable are the following :

1. A high degree of myopia. The continuous use of neutral- izing concave glasses with myopia of a greater degree than ^ is un- advisable, on account of the diminished acuteness of vision which generally co-exists. Even in high degrees of myopia, where circumstances make it necessary to wear spectacles continuously, the patients generally prefer those which do not completely neu- tralize the myopia. A continuous use of such glasses is sometimes impossible even for distant vision. This is perhaps because of the preponderating elongation of the eye in the direction of the visual axis. The sagittal diameter increases relatively more than all the others, so that lenses which correct the myopia for central vision are too strong for all objects in the periphery of the visual field. Under such circumstances it is often best to wear continu- ously spectacles which remove the far point to about 12 inches, and then to employ an auxiliary glass, about — ^2? ^^^ distant vision.

2. Diminution in the range of accommodation. We have already remarked that the myopic eye is naturally adapted to only slight efforts of accommodation, and all straining of accom- modation is regarded as hurtful to it. If now by neutralizing glasses we remove the far point to an infinite distance, we at the same time remove the near point and the whole range of accom- modation away from the eye, and the myope who formerly saw near objects distinctly without accommodation, can now do so only by accommodating. We demand in this case no more from the accommodation than the emmetropic eye does without difficulty ; but we ought first to satisfy ourselves that the accommodation of the myopic eye is able to respond to such a demand without injurious straining. We ought, for instance, never to compel continuous accommodation up to the region of the near point. Myopes who have worn neutralizing glasses from their youtli, are compelled to use weaker ones on account of the gradual with-