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Show 722 DR. G. L. JOHNSON ON THE SEAL'S EYE. [Nov. 21, would be the thickness of the media to be neutralized by the water. In most fishes' eyes (as w e are aware) tbe lens almost touches the cornea and is nearly spherical. O n taking the refraction of the living Seal's eye out of water, I was surprised to find that he possessed a myopia of 4 diopters or 10 in. in the vertical meridian and 13 diopters or about 3 in. in the horizontal,-the difference between these two curvatures producing an astigmatism of 9 D, an amount altogether incompatible with even useful sight and rarely if ever met with among human beings even in disease. O n applying a solution of atropine to the eye, I had an opportunity of ascertaining how the iris overcame this error. It is well known to oculists that high grades of astigmatism may be almost entirely neutralized by an opaque diaphragm in the centre of which is a narrow stenopaic slit placed at right angles to the error of curvature. I noticed the Seal's pupil contracted up to this shape, varying from a vertical slit 13 m m . long by 3 m m . broad, to one 3 | m m . Fig. 3. Fig. 4. Fig. 5. Tbe Seal's iris. Fig. 3 shows the Seal's iris with its narrow, vertical, slit-like pupil as seen out of water. Fig. 4 shows the pupil in a state of extreme contraction. Fig. 5 shows the pupil widely dilated by atropine and when under water. long and barely 1 mm. broad. By careful estimatiou of the refraction in the two opposite meridians by the direct method with the ophthalmoscope and by retinoscopy, I ascertained both eyes to have the same refraction in the two meridians and the meridian of greater myopia to be the horizontal, which could be corrected by a stenopaic slit in the vertical meridian. H o w does this extraordinary amount of astigmatism help the animal to see? The difference between vision in air and in water, viz. 26 D or 27 D, may be corrected to a large extent by powerful accommodation, and tbe 13 D of myopia may correct half the amount of error in the horizontal meridian. On the other hand, the vertical meridian will only be corrected to the extent of 4 D^ which is only a sixth or a seventh of the total. Moreover, I find that the Seal's iris dilates to the full in water |