November 



[917] 



NATURE 



165 



more or less divergent, these portions are not suffi- 

 ciently refracted to unite upon the centre, but reach 

 the surrounding parts of the retina in the order of their 

 refrangibility, red external, blue internal, yellow inter- 

 mediate. 



The cause of the colour phenomena, therefore, is 

 diminished refracting power of some of the ocular 

 media, and in this relation it is natural to think first 

 of the crystalline lens, on account both of the com- 

 plexity of its structure and of the well-known fact 

 that it is liable not only to lose its transparency and 

 elasticity in old age, but also to acquire a yellowish 

 or brownish tint. It has been assumed, but, so far as 

 I know, without evidence, that such colour changes are 

 of almost normal occurrence in old age ; and, some 

 eighty years ago, an ingenious quack traded upon the 

 suggestion that they were not only normal, but also 

 useful, and placed upon the market, at a high price, 

 spectacle lenses professedly made of clear amber and 

 supposed to be highly advantageous to old people. More 

 recently Dr. Liebreich amused the Royal Institution by 

 a lecture in which he maintained that the peculiarities 

 of Turner's later colouring were due to the gradually 

 deepening yellow of his crystalline lenses. I have, of 

 course, removed many yellow or brown lenses in cases 

 of senile cataract ; but I know of no evidence that the 

 healthy lens of an accurately seeing eye changes its 

 colour with age, and I believe that my own perception 

 of all shades of colour remains entirely accurate, 

 and affords satisfactory evidence of complete lenticular 

 colourlessness and transparency. 



The vitreous body does not, I think, either display 

 any change of colour as an incident of advancing 

 life, or take any active part in refraction, and 

 my observations lead me, at least in my own case, 

 to dismiss the corneas from consideration. My spec- 

 trum rings are too constant, and too uniform in size, 

 constitution, and colour, to be due to a structure liable 

 to be affected by atmospheric, secretory, or compress- 

 ive changes. I have kept my eyes open as long as 

 possible, have compressed them with and through my 

 eyelids, have rubbed the eyelids themselves, but, what- 

 ever I do, the colour rings remain unaltered. In a 

 word, I have fallen back upon the lenses themselves as 

 the immediate causes of the phenomena, and the ques- 

 tion that next arises is whether these phenomena justify 

 any apprehension of diminution or loss of lenticular 

 transparency — in other words, of cataract. I think 

 not. I have carefully examined my own eyes by look- 

 ing at various sources of light, and at white clouds, 

 tiirough- minute slits or minute circular openings in 



■metal discs, and I do not discover any traces of striae 

 of opacity. The usual shadows are cast upon the 



! retina by minute cells or particles in the ocular media — 

 the shadows so minutely 'described by the late Dr. Jago 



:in his book on "Entoptics" — but beyond these there 

 is nothing. 



I have come to regard the colour rings mainly as an 

 accidental result of unimportant lenticular conditions, 

 tfie effects of which are intensified by the use of electric 

 Kght, and which may be dismissed from consideration 

 flo far as the quality or the maintenance of vision is. 

 concerned. They appear only when the gaze is directed 

 towards the luminosity furnishing them; and they 

 may, I think, be wholly disregarded. I shall be happy 

 if my experience can afford relief from anxiety to any 

 contemporary or other person to whom such rings may 

 have caused uneasiness. R. Brudenell Carter. 



every kind of living substance is subject to two reci- 

 procal forms of change, the one constructive or 

 ■'assimilative," the other destructive or " dissimila- 

 tive." (These terms are nearly synonymous with 

 Gaskell's more characteristic, though not quite class- 

 ical terms, "'anabolic" and " katabolic") Every effec- 

 tive stimulus causes one or other of these changes, 

 and at any given instant the living substance is in a 

 state of unstable balance between the two, like a 

 flying animal or machine between the force of gravity 

 and the lifting force. On the cessation or diminution 

 of any stimulus, the living substance tends to return 

 towards the state of balance from which that stimulus 

 changed it. 



The theory applies especially to the very unstable 

 substances of muscle, nerve, and sense-organ. Now, 

 if an effective stimulus be removed from a sense- 

 organ, the return of the sensitive substance towards 

 the former state of balance, being a reciprocal change, 

 produces a reciprocal sensation if such be possible, as 

 when the removal of a hot body from the skin causes 

 a sensation of cold, or the removal of a coloured 

 object from the field of vision causes an after-image 

 of the complementary colour. So the cessation of the 

 stimulus of a moving image on the field of vision 

 causes reciprocal changes (of complex character, no 

 doubt) in the nerve-tissues concerned, which are inter- 

 preted at headquarters as reciprocal motion. 



An English translation (by Miss F. A. Welby) of 

 Hering's paper describing this most in'teresting and 

 important theorv mav be found in Brain, 1897, ,d. 232. 



F. J. Allen. 



Cambridge, October 20. 



Native Grasses of Australia. 



I regret that in my "Age of Mammals," published 

 in 19 10, the statement is erroneously made that native 

 grasses are absent from Australia. I am unable to 

 find my authority for this statement, and I regret that 

 it has been quoted in a recent text-'book of geolc^y 

 by my friend. Prof. H. F. Cleland. 



Prof. E. W. Berrv, botanist at Johns Hopkins Uni- 

 versity, informs me as follows: — "There are certainly 

 plenty of native grasses in Australia; in fact, there is 

 quite a large number of genera confined to that coun- 

 try or to Australia and New Zealand, which is un- 

 usual for this group, since grasses, as shown by their 

 present distribution, are an old stock, and enjoyed a 

 nearly world-wide radiation probably as earlv as the 

 Upper Cretaceous. Possibly the multiplication of turf- 

 forming species was not accomplished until the pro- 

 gressive desiccation of the climate in certain areas at 

 a later time, and I think that this distinction has been 

 more or less overlooked. Some of the genera of 

 grasses confined to Australia are : — Neurachne, Plagio- 

 setum, Xerochloa, Potamophila, Microlaena, Tetrar- 

 rhena, Amphipogon, Echinopogon, Dichelachne, Diplo- 

 pogon, Pentapogon, etc." 



Henry Fairfield Osborn. 



The American Museum of Natural History, 

 New York, September 27. 



An Optical Phenomenon. 



The phenomenon described by Capt. C. J. P. Cave 

 ^ature, October 18, p. 126) is one of the many in- 

 mces which support Hering's "Theory of the Pro- 

 Bses in Living Substance." According to this theory 

 NO. 2505, VOL. 100] 



Vegetable Pathology and the Vicious Circle. 



In animal pathology disease is frequently compli- 

 cated by reactions which aggravate the primary morbid 

 process, and so establish what is known as a "vicious 

 circle." This process vires acquirit eundo, and may 

 lead to the perpetuation of disease, to the destruction 

 of an organ, or even to the termination of life. I 

 should be glad to know whether any examples of such 

 "vicious circles" are met with in vegetable pathology. 

 Jamieson B. Hurry. 



Westfield, Reading, October 26. 



