722 



DB. G. I/. 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 we are aware) the lens almost touches the 

 cornea and is nearly spherical. 



On 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 incom- 

 patible with even useful sight and i-arely if ever met wdth among 

 human beings even in disease. 



On applying a solution of atropine to the eye, I had an oppor- 

 tunity 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 mm. long by 3 mm. broad, to one 3| mm. 



Fig. 3. 



Fig. 4. 



Fig. 5. 



The 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 estimation 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. 



How 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 the 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 



