292 



NATURE 



[July lo, 1903 



Hering had shown that this phenomenon is a function 

 of the condition of dark-adaptation produced by feeble 

 illumination rather than a function of the feeble 

 illumination itself, and von Kries shows that the 

 changes of relative brightness are readily explicable if 

 we suppose that, as the eye becomes more and more 

 dark-adapted, there comes into play a new factor which 

 has no influence, or no appreciable influence, at 

 ordinary luminosities. Speaking roughly, the blue 

 end of the spectrum becomes relatively brighter, and 

 it is this end of the spectrum which has the greater 

 action on visual purple. 



In pronounced dark-adaptation the spectrum is seen 

 as a colourless band of light, and the curve of lumin- 

 osity of the spectrum in this condition shows a close 

 correspondence with the curve representing the degree 

 of action of different parts of the spectrum on visual 

 purple. The spectrum is shortened at the red end; it 

 is brightest in the green, and the diminution of bright- 

 ness towards each end is much more gradual on the 

 Jblue than on the red side of the maximum. 



Visual purple also furnishes an explanation of an 

 anomaly of colour vision which has long puzzled 

 physiologists. A colour-equation which is good for 

 one luminosity is not good for all luminosities, and 

 von Kries shows that the mixed light which becomes 

 relatively brighter at low luminosities is that which 

 has the greater action on visual purple. 



The absence of visual purple from the jovea centralis 

 provides a ready method of putting the theory to the 

 test. If dark-adaptation with its influence on colour-' 

 -brightness and colour-equations be due to visual 

 purple, the fovea should not share in the increased 

 sensitiveness of the dark-adapted eye, nor should this 

 -region show any change in colour-brightness or in 

 colour matches in different conditions of adaptation. 



There seems to be no doubt that the fovea responds 

 in favour of the theory. There is some difference of 

 ■opinion as to whether this region fails entirely to show 

 alteration of sensitiveness, but it is generally agreed 

 that any increase which occurs is insignificant com- 

 pared with that of the surrounding region of the retina. 

 Very careful observations by Nagel and others seem 

 also to show conclusively that Purkinje's phenomenon 

 -and the alteration of colour-matches are absent if the 

 stimulation of the retina be strictly limited to the 

 foveal region. The features of colour vision which 

 .are believed to depend on visual purple are absent just 

 when, according to the theory, they should be absent. 



One of the most interesting developments of the 

 theory is that in which the condition of total colour- 

 blindness is regarded as vision dependent chiefly, or 

 exclusively, on the visual purple of the rods. Hering 

 was the first to show that the curve of luminosity of 

 the spectrum in most cases of total colour-blindness 

 corresponds with great exactness to the curve of 

 luminosity of the normal dark-adapted eye, and von 

 Kries shows that there are other points of close re- 

 semblance between the two conditions. 



If visual purple be the basis of monochromatic 

 vision, there ought to be a central blind spot, and in 

 several cases which have been examined from this 

 point of view by quite independent observers, this has 

 been found to be the case. Again, the behaviour of 

 NO. 1 76 1, VOL. 68] 



the fovea is in favour of the theory. The evidence 

 here, however, is not unanimous. Hess has failed to 

 demonstrate the existence of a central scotoma in 

 several cases, but our knowledge of the exact distribu- 

 tion of rods and cones in'fhe human fovea is based on 

 very few examinations, and it is possible that there 

 are wide individual variations, and that in some people 

 a.i area devoid of rods may be absent, or so small that 

 it is impossible to demonstrate its presence. The 

 diffusion of visual purple into the rod-free area is also 

 possible in some cases, but it seems more probable, 

 from a study of the evidence as a whole, that there are 

 two kinds of total colour-blindness, and that in only 

 one of these is it probable that visual purple is the only 

 sensitive substance in the retina. 



Several of the papers in the " Abhandlungen " deal 

 with the recurrent image, or " ghost " of Bidwell, 

 which is believed by von Kries to be a " visual purple " 

 phenomenon. This part of von Kries 's work has been 

 much attacked, and recent work seems to show that 

 the recurrent image is a much more complex pheno- 

 menon than has usually been supposed. It is probable 

 that visual purple is only the basis of one of the 

 elements of the complex. 



The comparative evidence is in favour of the theory, 

 visual purple being abundant in nearly all vertebrates 

 the habits Of which are nocturnal or which live under- 

 ground. 



It has only been possible here to give the briefest 

 sketch of the views of von Kries and his co-workers. 

 The " Abhandlungen " should be consulted for the 

 elaborate investigations and detailed arguments in 

 support of their views. W. H. R. Rivers. 



OUR BOOK SHELF. 



Botanische Forschungen des Alexander zuges. By Dr. 

 H. Bretzl. Pp. xii + 412. (Leipzig: Teubner, 1903.) 

 Price 12 marks. 



The criticism passed by Sachs in his " History of 

 Botany " on the writings of the ancient classical 

 writers, including Theophrastus, seems to have been 

 unnecessarily severe where he passes over their " cor- 

 rupt texts " with a brief mention. At that time the 

 study of geographical and ecological botany had not 

 received the stimulus which was mainly induced by 

 the appearance of Schimper's master work, " Die 

 Pflanzengeographie. " It would hardly be going too 

 far to say that it required the development of this 

 branch of the subject to admit of the full appreciation 

 of Theophrastus 's work. For the essential feature of 

 Theophrastus 's " Plant Geography," and this book is 

 the main source of information concerning Alexander's 

 expedition, is the painting of a series of word pictures, 

 illustrations of types of vegetation, in which, while 

 correct morphological ideas could hardly be looked for, 

 the descriptions, in their accuracy of observation and 

 power of expression, are not often excelled by those 

 due to present-day writers. As might be expected, 

 some of the accounts are difficult of explanation, and 

 discrepancies arise which have demanded considerable 

 skill and enthusiasm on the part of Dr. Bretzl to clear 

 up. Others are more obvious; thus the paragraph 

 which begins : — 



" vnoj3e^p(oTai Se ravra ra dfvSpa navra Kara fieaov vtto ttjs 

 6dXdTTr]s Koi ea-rrjKtv vtto tSjv pi^o)v &cnr(p no\vnovs" calls 

 up very definitely the picture of a mangrove swamp. 



