April 6, 1899] 



NATURE 



541 



again there is no note of warning. Assuming, for the 

 moment, that floats give a trustworthy record of the 

 movement of the surface water in which they are im- 

 mersed, they give little information about the real motion 

 •of any body of water in their course. A single example 

 must suffice to illustrate this. Set a float adrift in the 

 <;ulf of Mexico, and it is found in the Shetland Islands ; 

 we cannot conclude that water has come from the Gulf 

 ■of Mexico to the Shetlands — the current which brought 

 ■the float to the banks of Newfoundland may have there 

 become an under-current, and the float cannot follow it ; 

 it remains on the surface, and is borne eastward by 

 ■water which may have come from Davis Strait or 

 Denmark Strait, where no floats are set adrift. No 

 ■doubt the fact that the float took the course it did is in- 

 teresting, and with sufficient knowledge of the mechanism, 

 obtained from other sources, it may be of great value ; 

 but even in a simple case, like that quoted, the greatest 

 -caution must be exercised, and the uncertainty becomes 

 still greater in channels and enclosed seas. 



It seems fair, from Dr. Schott's discussion, to conclude 

 that observations by means of floats are likely to con- 

 tribute valuable additions to our knowledge of the surface 

 movements of the waters of the sea, when taken in con- 

 junction with systematic observations by more precise 

 methods, of which the distribution of temperature and 

 •salinity is probably the most satisfactory. Taken by 

 themselves, the float observations are liable to be 

 •jeriously misleading. H. N. D. 



A COMPARATIVE STUDY OF VISUAL 

 ACCOMMODATION. 



SO recently as five years ago but little was known with 

 certainty regarding the refraction and accommoda- 

 tion in animals with " camera eyes." It is Beer's ' credit 

 to have made an exhaustive investigation of a large 

 number of animals with the aid of all modern ophthalmic 

 methods, in addition to an experimental method of 

 Tiis own devising, namely, electrical stimulation of the 

 enucleated eye. 



No mechanism for accommodation is known in the 

 facetted eyes of crabs and insects ; according to Exner 

 it is not required. The retina is comparatively thick, and 

 moving an object from So cm. to i mm. from the eye 

 causes an alteration of but '01 mm. in the position of the 

 ■image. 



A similar argument was supposed to hold good for the 

 rabbit's eye, even by such a distinguished physiologist as 

 Magendie ; but this was due to an experimental error. 

 'Kepler, although ignorant of accommodation, recognised, 

 nevertheless, that the image on the retina must be sharp 

 for visual purposes. 



Speaking generally, an eye must be able to accom- 

 siiodate proportionally to (i) its own size, (2) the width 

 of the pupil, (3) the closeness of the retinal mosaic. 

 Whilst, therefore, an emmetropic man can see plainly at 

 6 m. without exercising his accommodation, those animals 

 — and there are many — with much larger eyes must, for 

 the same acuity of vision, already accommodate at that 

 distance. On the ot'ner hand, it is not so important for 

 large animals to have so near a "near-point" as small 

 animals. Small animals, e.g. birds, have to see objects 

 clearly even when quite close ; they require a large range 

 of accommodation. Larger animals, except monkey and 

 man, who bring things close to the eye with the hand, 

 can get on with a "near-point" of \-\ m., corresponding 

 to a range of accommodation of 1-2 diopters ; this is so 

 with the horse and ruminants. 



Three modes of accommodation are possible : (i) alter- 

 ation in the refractive power of the various media ; (2) 



1 " The .-Vccoraniodalion of the Eye iii the .-Animal Kingdom." By Th. Beer, 

 Lecturer on Comparative Physiology in the University of Vieiin.i. 



NO. 1536, VOL. 59] 



alteration of the curvature of the refracting surfaces ; 

 (3) alteration of the relative position of lens and retina. 



This first method, by which Grimm in 1785 sought to 

 explain accommodation, has never been actually observed. 

 The last was formerly a favourite theory. It was taken 

 up by Kepler and by Scheiner. Many absurd reasons 

 were given, and mechanical hypotheses constructed by 

 various individuals to support this theory ; nobody thought 

 of examining the actual facts. In certain classes of 

 animals, viz. cephalopoda, fishes, amphibia and reptiles, 

 accommodation is effected by alteration of the distance 

 between lens and retina. In man, however, as in all 

 mammalia, also in birds, lizards and tortoises, this is 

 brought about by alteration in the curvature of the lens. 



It is also obvious that active accommodation may be 

 for a near (positive) or for a far (negative) point, viz. the 

 resting eye may be adjusted for distance or proximity 

 respectively. 



The dibranchiate cephalopoda are the only inverte- 

 brates in which accommodation has been observed; and, 

 although they have " camera eyes," their type, as a whole, 

 is far inferior to that of vertebrates. By retinoscopy 

 it was determined that their eye is normally adjusted for 

 the near-point. The extent of this normal myopia varies 

 between two and (as an extreme limit) ten diopters. The 

 mechanism of accommodation is as follows: — The bulb 

 forms half of a rough ellipsoid. At the equator is a flat, 

 strong cartilaginous ring, separating the anterior flattened 

 portion of the eye from the posterior ellipsoidal part. 

 Behind the ring the sclerotic is comparatively soft and 

 yielding. In the anterior wall of the eye is also a strong 

 muscular ring with radial fibres running from the carti- 

 laginous ring to the ciliary body, which is firmly attached 

 in the equatorial region of the lens. When this muscle 

 contracts it pulls back the whole anterior wall of the eye, 

 including the ciliary body and lens, towards the interior 

 of the eye. The resulting increased pressure would tend 

 to make the bulb of more spherical shape, with consequent 

 increase of distance between lens and retina, but the 

 thinner consistency of the posterior half leads to an actual 

 diminution of the antero-posterior diameter, and conse- 

 quent approximation of retina to lens. The iris, which 

 lies practically outside the bulb, and also serves as lid, 

 although extremely sensitive to light, takes no part in 

 accommodation. 



Teleostean fishes, although the structure of their eye 

 bears a superficial resemblance to that of the cephalopod 

 eye by virtue of the spherical form of the lens, but is in 

 other respects vertebrate in structure, are, nevertheless, 

 myopic and accommodate for distance. The thickness of 

 the retina,with its sensitive layer on the outer side (in the 

 cephalopoda it is on the inner side), introduces a factor 

 which has to be taken into account when estimating the 

 refraction. Thus an apparently hypermetropic eye may 

 prove to be really myopic. The comparative opacity of 

 water, which does not allow of distinct vision for any 

 great distance, accounts for their permanent myopia, but 

 their range of accommodation is sufficient to enable fishes 

 to focus parallel rays on the retina. But since their 

 cornea is not, as was formerly supposed, flattened, when 

 taken out of water they are so highly myopic that the 

 correction which they possess would be of no value or 

 account. The mechanism of the adjustment is different 

 to that of the cephalopoda, and quite different to that of 

 the vertebrata. 



Ciliary body, ciliary muscle, zonula, spaces of Fontana 

 do not exist in fish-eyes, and the iris does not glide on the 

 lens, but is generally quite free. The spherical lens is 

 suspended from above by a strong triangular band ; below 

 is the structure known as the "companula," which, from 

 its function. Beer re-names the " retractor lentis "; it draws 

 back the lens nearer to the retina during accommoda- 

 tion. This can be seen on electrical stimulation of the 

 recently enucleated bulb, even after removal of the cornea. 



