February 3, 1888.] 



SCIENCE. 



59' 



wing for days and nights together without intermission ; many 

 tishes require perpetual motion in order to preserve their equiUb- 

 rium, while other pelagic forms appear to be on the move for long 

 periods of time without flagging, — all these cases necessitating oft- 

 repeated movements, which call for far more serious strain on the 

 muscles than the mere extension of the wings during the act of 

 soaring. 



The strain on the extensor muscles at such a time can be but 

 trifling, compared to the strain on the levators and depressors of 

 such a bird as the albatross, whose weight of nine to fifteen pounds 

 is supported by two levers of the third class, five to seven feet in 

 length ; and yet no bird makes longer flights than this wanderer of 

 the southern seas, who has no special device to keep his wings out- 

 stretched. 



These instances are brought forward, not to disprove the fact 

 that a device to ease the muscles in soaring may not exist, but to 

 show that there is apparently not the slightest need for it. 



In regard to the interlocking of the primaries, which unquestion- 

 ably takes place, is not this the result of their emargination, and 

 consequent failure to glide smoothly over one another, rather than 

 the end to be accomplished by this cutting-away of the feather to- 

 ward the extremity .'' 



This view of the case is borne out by the fact that the longer, 

 more flexible ulnar border of the primary naturally gives at each 

 stroke of the wing, thus catching in the radial portion of the feather 

 immediately behind it, whether the bird wishes it or not. 



Moreover, during the act of soaring, the wing is expanded to its 

 utmost, and the tips of the primaries widely separated, while in a 

 fresh specimen of Buieo borealis no locking is possible until the 

 wing is partially closed. This would seem to be conclusive as re- 

 gards the importance of the locking of the primaries as an aid in 

 soaring ; although there remain the facts that some birds who soar 

 to perfection — such, for example, as gulls, cranes, storks, and the 

 frigate-bird — do not possess emarginate primaries, while others, 

 like some owls and flycatchers, have emarginate primaries but do 

 not soar. 



Professor Trowbridge's comparison of the wing to a flat card is 

 hardly felicitous, and his statement that it would be in a state of 

 unstable equilibrium but for the locking of the primaries would seem 

 open to serious doubt. 



One absolute requisite of a wing is that the anterior margin 

 should be rigid, and the posterior border flexible, — a requirement 

 which is met toward the extremity of a bird's wing by bringing the 

 quill close to the radial margin of the feather, leaving a posterior 

 pliable edge. 



Now, if the primaries are interlocked, a rigidity is created toward 

 the ulnar border of the wing, which would thus become more card- 

 like and unserviceable than if the primaries did not lock. 



A pertinent question that might be asked of Professor Trow- 

 bridge, is. Why, if the " long primaries present a serious resistance 

 . . . when a bird is soaring," do all birds that soar or sail possess 

 just such primaries, while the corresponding feathers in birds 

 which do not soar are short .' 



One feature in the wings of birds pre-eminent for soaring abili- 

 ties, e.g., the VulturidcE and FalcotiidcR, has not been touched upon 

 in this discussion, so far as I am aware ; and this is the fact that 

 when the wing is extended to its utmost, as it invariably is during 

 soaring, the metacarpus and phalanges are not in line with the 

 ulna, but are bent forward of it. By this arrangement some of the 

 muscles and tendons that ordinarily act in flexing the wing are 

 brought upon the dorsal surface of the bones, and thus have their 

 power of flexion weakened, or possibly even made to aid in the au- 

 tomatic extension of the wing. If, now, a bird with wings thus 

 spread be so killed that there is no perceptible shock or nervous 

 start, the bird may remain with outstretched pinions and sail gradu- 

 ally downward, — exactly such a case as Professor Newberry de- 

 scribes. 



In conclusion, I can but regret that I have no facts to ad- 

 duce that will throw any light on the problem of flight, as it is 

 far easier to find fault with any theory than to suggest a better, and 

 purely adverse criticism must always seem more or less ungracious. 



Frederic A. Lucas. 



Washington, D.C., Jan. 16. 



Binocular Combinations upon Disparate Retinal Points. 



Every one is familiar with the fact that Wheatstone and many 

 subsequent investigators have explained the binocular perception 

 of solidity by the theory of the ' fusion of images upon disparate 

 points,' as they are called, in the retina. They have generally denied 

 the original possibility of a monocular perception of solidity and 

 distance ; and hence, when certain plane figures were stereoscopi- 

 cally combined, the apparent solidity of the resulting single figure- 

 suggested its explanation in accordance with what had previously 

 been supposed of the mathematical relation between combination 

 and convergence. Thus Wheatstone's view may be illustrated by 

 the following figure. It is well known that the stereoscopic combi- 

 nation of these figures, although making a plane image only upork. 

 the retina and representing only a plane surface externally, never- 

 theless produces the appearance of a solid body. Previous theories 

 of vision had maintained that single vision took place upon corre— 



\ 



\ 





//' 



/ 





k^ 



spending points of the retina, and double vision upon disparate 

 points. Now, as the mathematical construction of the case would 

 not allow the inner figures and lines to fall upon exactly corre- 

 sponding points, the apparently single character of the image in 

 stereoscopic combination was most naturally explained by saying 

 that fusion took place upon disparate points ; and hence when the 

 perception of solidity, or relatively different distances between the 

 larger and smaller figures, uniformly accompanied this kind of 

 fusion, it was naturally ascribed to that process as its cause. 

 Whether such a fusion really takes place or not, has been hotly- 

 contested, and we wish here to present a few new considerations- 

 to show that it does not occur, notwithstanding the strongest ap- 

 parent evidence of our actual perception of it. 



To make the argument clear, a few words will be necessary upon 

 what is meant by ' corresponding ' and ' disparate ' points. As in- 

 dicated, they denote the points upon which respectively single and 

 double vision takes place. But the second term has two very dis- 

 tinct applications, — one binocular, and the other monocular. It is- 



this last fact and its implications which most investigators, and 

 among them Wheatstone, seem to have ignored. But the impor- 

 tance of taking it into account will be evident from the following 

 considerations. Take the circles R and L to represent the retinal 

 surfaces of the two eyes. Divide each retina into halves by the 

 vertical meridians AB and A B' . Draw also the horizontal merid- 

 ians in which lie the points a, b, c, d, e, and a' , b' , c' , d' , e' ; c and 

 c\ at the intersection of the vertical and horizontal meridians, repre- 

 sent the fovea centralis of each eye. Now, the vertical meridian 

 divides each eye into halves, that correspond to the opposite halves 

 of the other eye. Thus we have what are called the nasal or inner, 

 and the temporal or outer halves of the eyes. The nasal halves of 

 each eye are said to ' correspond ' to the temporal halves of the 

 other eye. How this will appear can be seen by superimposing 

 one circle upon the other ; and the points a and b in the tem poral 

 half of the left eye, Z, will coincide with a! and b' in the nasal half 

 of the right eye, R ; and rfand e in the nasal half of the left wilt 

 coincide with d' and e' in the temporal half of the right eye. By 



