iti6 



NA TURE 



[June 22, 1695 



for the most part the inherited tendencies of cell growth 

 acquired as legacy from the canal organs of the surface." 

 Among important topics which the author ignores, and upon 

 which we could have wished his opinion, are Chatin's 

 alleged discovery of all intermediate stages between the 

 rod-bearing and ciliated cells of the Batrachian auditory 

 epithelium, and the views of Engelmann, Chun, and Yves 

 Delage, arising from the experimental study of the otoliths in 

 Ctenophora, Mollusca, and Crustacea. The latter are by 

 no means reconcilable with the author's bold assertion that "the 

 functions of the otoliths are entirely unknown." In dealing with 

 the " chalk-sacs'' of the amphibia, the author remarks (p. 2l) 

 that their " morphological as well as physiological significance " 

 is still unknown. He ignores the fact that Lenhossek has shown 

 them to be tubular glands and named them ; and this is very 

 remarkable, as, while he makes no mention of that author's 

 paper, he acknowledges one by Coggi, in which it receives 

 ample recognition. G. B. H. 



PERSPECTIVE AND COLOUR. 



TN Bi-ain, Paris LXI. and LXIL, which have just been 

 published, occurs an interesting article by Prof. Einthoven 

 (of Leyden) on the production of shadow and perspective 

 effects by difference of colour. The following is an account 

 of the phenomena : — 



Difference of colour may, under certain circumstances, be 

 the cause of an apparent difference in distance.' To observe 

 the phenomenon, it is only necessary to glue different coloured 

 figures, such as letters of blue and of red paper, to a screen of 

 black velvet and to look at them from a suitable distance. In 

 the experiment about to be described, Roman capital letters of 

 about eight by four centimeters were used, the .=creen being 

 placed at about three meters distance from the observer.^ 



Under these conditions it appeared, both to Prof. Einthoven 

 and to others who he interrogated, that the red letters were nearer 

 than the blue. Obviously, the phenomenon might be explained 

 by difference in accommodation. In order to see the red letters 

 distinctly, a greater amount of accommodation is necessary than 

 in focussing the blue ones, and the greater sense of effort might 

 account for the notion of the red letters being nearer. This 

 accommodation hypothesis, plausible as it seems, cannot how- 

 ever be accepted asasatisfactory explanation of the phenomenon. 

 Several observations tell against it, notably this : that there are 

 about as many persons who see the blue letters before the red, 

 as there are those who see red before blue. In the second 

 place, the apparent difference in distance — so distinct to binocu- 

 lar vision — disappears almost wholly with the closure of one 

 eye. Looked at wilh one eye only, and for some length of 

 time, the letters appeared to be lying in the same plane, but 

 each time that the other eye was opened the difference in 

 distance obtruded itself irresistibly. 



The amount of difference remains constant, and can be 

 estimated with considerable accuracy, in the same way as in 

 making a stereoscopical observation. The question therefore 

 suggested itself, whether we had not here to deal with real 

 stereoscopy ? The answer to this question is an affirmative. 

 Briicke-' has shown by means of a simple experiment that the 

 retinal images of differently coloured points are shifted with 

 respect to one another. Looking with one eye at a narrow 

 vertical strip on a black background, the upper and lower thirds 

 of the strip being red and the middle third blue, Briicke ob- 

 served that the blue part deviated to one side, the two red parts 

 to the other side. By covering either eye alternately, a deviation 

 of the blue and red parts in opposite directions will be observed ; 

 and, on both eyes being used, the notion of a difference in 

 distance is proved by the combination of the two images in 

 such a way that the parts that deviate to the nasal side constitute 

 the nearer image ; the parts that deviate to the temporal side, 

 the farther image. The stereoscopical effect is, however, more 

 distinct and convincing with the coloured letters than wilh the 

 strip used by Briicke. 



The cause of the relative removal of the differently coloured 

 images lies in the eccentricity of the pupil, as may be demon- 



1 Donders. M'etensch. bijbladen. Med. Gastli. v. OogUjders, 1868. 

 - W. Einthoven," Stereoscopic dependant d'une difference decouleur." 

 Archives Necrlandaises. t. 20. 



•' Vorlesungen iiber Phypiologie Wien, 1884, 3 Aufl. B. 2, S. 93- 



NO. 1234, VOL. 48] 



strated experimentally. The pupils may be made highly 

 eccentric by covering them partially. Partial covering on the 

 nasal side is equivalent to a removal of the pupil to the 

 temporal side, and conversely, covering the temporal side is 

 equivalent to removal to the nasal side. With a nasal eccentric 

 pupil a shifting of the differently coloured images in one direc- 

 tion will be observed ; with a temporal eccentric pupil the 

 shifting will be in the other direction. 



The effect of an artificial eccentricity of the pupil is surprising 

 when both eyes are used. Anyone who sees the red letters 

 before the blue has only to cover his pupils symmetrically on 

 the temporal side, when he will observe the red letters retreat 

 and soon appear to be behind the blue. On covering his pupils 

 symmetrically on the nasal side, the red letters come forward 

 more and more, and seem at last (experimenting at a distance 

 of four or five meters) to remain several decimeters in front of 

 the blue. A person who sees the blue letters before the red, 

 has only to cover his pupils on the nasal side, when he will 

 observe that the distances change, the red coming forward and 

 the blue shrinking back. 



Lately, however, Ur. A. D. Waller has found that on re- 

 peating the experiment with a seemingly slight modification, 

 he obtained the same effects with one eye alone. He used as a 

 test object rings of blue paper on a red ground, or of red paper 

 on a blue ground, and found that the nasal pupil of the left 

 eye gives the same appearance of circular trenches or hillocks as 

 does the temporal pupil of the right eye. 



This observation has been the motive to a more thorough 

 study of the phenomenon. 



On looking with the right eye and a temporal pupil at red 

 rings on a blue paper, the rings appear as circular hillocks 

 when the paper is held to the left, and also sloping in that 

 direction. One seems to be looking against the dark edges 

 of a thick red ring fixed upon the blue paper. With a nasal 

 pupil the red rings appear as circular trenches. 



The phenomenon is the more striking in proportion to the 

 purity of the colours used. The pupil must be made sufficiently 

 eccentric and in a suitable direction by means of a black screen 

 that covers it from one side, or better still, by meansof astenopaeic 

 apparatus. The pupil must not be too narrow, and the whole 

 eye should be wide open and well-directed, so as to avoid any 

 partial covering by the nose, eyelid, or lashes. Lastly, it is not 

 desirable to keep the eccentricity of the pupil constant for more 

 than a brief period. For if one stares at the rings a long time 

 with unmoved pupils, all appreciation of distance is lost, as in 

 so many cases where only one eye is used, and the rings may 

 even seem to lie in a plane that intersects the plane of the blue 

 paper perpendicularly. If, on the contrary, one shift; the 

 screen or the stenopteic apparatus now and then the rings appear 

 to rise and sijik, and, under the above-mentioned conditions 

 the rising will be with temporal pupil, the sinking with nasal 

 pupil, and in a way almost as striking as if they were seen 

 stereoscopically. 



Prof. Einthoven proves mathematically that the explanation 

 of the phenomenon is found in the appearance of shadows. 



THE FLORA OF GREENLAND. 



TN 1891 Dr. William H. Burk accompanied, as botanist, the 

 party which escorted Lieut. Peary to his winter quarters in 

 McCormick Bay. A number of plants were collected and taken 

 to America, but they had barely been determined before an 

 expedition was organised to search for Lieut. Peary, and Mr. 

 William G. Meehan was appointed botanist to it. This was 

 just a year ago. Mr. Meehan was also fortunate enough to 

 obtain specimens, and a catalogue of the plants collected in both 

 cases was communicated to the Academy of Natural Sciences of 

 Philadelphia on April II. Some idea as to the character of 

 the catalogue will be obtained from the following introduction 

 to it :— 



The range of territory covered by Dr. Burk and Mr. Meehan 

 was between about latitude 63° and above 78° or between 

 Godthaab and Littleton Island. 



As nearly the » hole collection was repeated by each collector, 

 it may be taken as a fairly complete flora of that portion of the 

 territory of Greenland. 



Before starting in their respective journeys, both Dr. Burk 

 and Mr. Meehan were instructed to examine as far as possible 



