596 



NATURE 



[Feb. I, 1877 



wave-length 503, and is probably that described by Cornu as 

 sixth in order of brightness, at wave-length 500. At the end of 

 last month the brightest line was about 484, probably the F 

 hydrogen line. Since December 27 the new star has always ap- 

 peared to me orange. Has not this star received any name yot ? 

 Sunderland, January 26 T. W. Backhouse 



KUHNE'S RESEARCHES ON PHOTO-CHEMI- 

 CAL PROCESSES IN THE RETINA 

 ON January 5, Dr. W. Kiihne, Professor of Physiology 

 in the University of Heidelberg, read before the Natur- 

 historisch-Medicinisches Verein, of Heidelberg, a paper entitled 

 " Zur Photo-chemie der Netzhaut," so full of interest to the phy- 

 sicist and physiologist, that T think an abstract of it will be 

 acceptable to the readers of Nature. 



A short time since. Boll (a pupil of Max Schultz and Du Bois- 

 Reymond, who novf occupies the chair of Physiology in Rome) 

 communicated to the Berlin Academy the remarkable fact that the 

 external layer of the retina, i.e., the layer of rods and cones, 

 possesses in all living animals a purple colour. During Kfe, 

 according to Boll, the peculiar colour of the retina is perpetually 

 being destroyed by the light which penetrates the eye ; darkness, 

 however, restores the colour, which vanishes for ever almost 

 immediately after death. ' 



The wonderfully suggestive nature of Boll's discovery led 

 Kiihne to repeat his observations ; in doing so, whilst he has 

 confirmed the fundamental statement of Boll, he has ascertained 

 a number of new facts of great interest. 



Kiihne's observations were made on the retinse of frogs and 

 rabbi. s. In the first place, implicitly relying upon the statements 

 of Boll, he examined, as soon as possible after death, the retina 

 of animals which had been kept for some time in darkness. He 

 soon found that the beautiful purple colour persists after death 

 it the retina be not exposed to light ; that the bleaching takes 

 place so slowly in gas-light, that by its aid the retina can be pre- 

 pared and the changes in its tint deliberately watched ; that 

 when illuminated with monochromatic sodium light the purple 

 colour does not disappear in from twenty-four to twenty- eight 

 hours, even though decomposition have set in. 



These first observations of Kiihne on the vision-purple {SeJipur- 

 pur), as he terms it, whilst they showed that the disappearance of 

 the colour is not, as Boll had asserted, a necessary concomitant of 

 death, removed many of the difficulties which stood in the way 

 of a careful investigation. Carrying out his preparations in a 

 dark chamber illuminated by a sodium flame, Kiihne was able to 

 discover the conditions necessary to the destruction of the vision- 

 purple as well as some facts relating to its restoration or renewal. 



As long as the purple retina is kept in the dark or is illumi- 

 nated only by yellow rays, it may be dried upon a glass plate 

 without the tint changing ; the colour is not destroyed by strong 

 solution of ammonia, by saturated solution of common salt, or 

 by maceration in glycerine for twenty-four hours. On the other 

 hand, a temperature of 100° C. destroys the colour, and alcohol, 

 glacial acetic acid, and strong solution of sodiumTiydrate produce 

 the same effect. 



Kiihne's next observations were directed to the discovery of 

 the influence of light of different colour upon the vision -purple. 

 It would appear that the more refrangible rays of the spectrum 

 have the greatest action, and that the red rays are as inactive as 

 the yellow. 



Kiihne now found the incorrectness of Boll's assertion that 

 the retina of the living eye exposed to ordinary daylight does 

 not exhibit the vision- purple, for on preparing the eyes of ani- 

 mals which had just been exposed to light, as rapidly as possible 

 in the chamber illuminated by sodium light, he discovered that 

 the retina was of a beautiful purpie. It was only when eyes 

 were exposed for a considerable time to the direct action of the 

 sun's rays that a fading of the purple colour was perceived. 



A most suggestive experiment now threw some light upon 

 the circumstances which retard the decolorisation, and which 

 restore the vision-purple. The two recently extirpated eyes of a 

 frog were taken ; from one the retina was removed, whilst an 

 equatorial section was made through the other eye, so as to 



expose the retina and still leave it in situ. Both preparations 

 were exposed to diffuse daylight, until the isolated retina had 



' This account of Boll's researches is taken from Kiihne's paper. The 

 latest number of the Monatsberichte of the Berlin Academy which has yet 

 reached Manchester, which includes the Proceedings for September and 

 November, does not contain Boll's communication, which is of later date 

 (November 12). 



lost its purple colour. On now taking the other preparation 

 into the yellow chamber and removing the retina, it was found 

 that its colour yet remained : it was dark red, but was bleached 

 when exposed in its naked condition to daylight. 



This experiment was confirmed by others, in which the effect 

 of strong sunlight was substituted for that of diffuse daylight. 



But the most curious results of Prof. Kiihne's experiments have 

 reference to the restoration of the vision-purple. If an equatorial 

 section be made through a recently extirpated eye, and a flap of 

 retina be lifted up from the underlying choroid and exposed to 

 light, the purple colour of the flap will be destroyed, whilst the 

 colour of the rest of the retina persists. If, however, the bleached 

 portion of the flap be carefully replaced, so that it is again in 

 contact with the inner surface of the choroid, comple restoration 

 of the vision-purple occurs. This restoration is a function of the 

 living choroid, probably of the living retinal epithelium {i.e. of 

 the hexagonal pigment cells, which used formerly to be described 

 as a part of the choroid), and it appears to be independent of the 

 black pigment which the retinal epithelium normally contains. As 

 it is absolutely dependent upon the life of the structures which 

 overlie the layer of rods and cones, it is natural that it should be 

 observed to occur for a longer time after somatic death in the 

 frog than in the rabbit. 



Kiihne's researches, though suggested by the interesting obser- 

 vation of Boll, have not only corrected many errors which that 

 observer had committed, but have led to the discovery of facts 

 which add immensely to the importance of the newly-observed 

 vision-purple. 



They have shown that the living retina contains a sub- 

 stance which under the influence of light undergoes chemical 

 changes, which vary in intensity according to the intensity and 

 character of the luminous rays, and they point to the existence of 

 structures in connection with the retina which as long as they are 

 alive are able to provide fresh stores of substance sensitive to light. ^ 

 Since the above account of Kiihne's researches was written, he 

 has published in the Centralblatt der medicinischen Wissenschaften 

 (January, 1877, No. 3) a short paper, dated January 15, in which 

 he announces the startling confirmation to his previous researches 

 afforded by his having bee?t able to obtain actual images on the 

 retina which corresponded tvith objects which had been looked at 

 during life (!). 



The discoveries of Boll and Kiihne must, as the latter remarks, 

 have led to the thought that after all there might be some truth in 

 the stories which we all have heard of images of things seen in death 

 being left imprinted upon the eye. After his first researches 

 Kiihne endeavoured over and over again to observe on the retina 

 of rabbits bleached spots corresponding to the images of external 

 objects, but his endeavours failed. As Kiihne remarks, and as 

 all readers who have understood his experiments will allow, in 

 order to obtain a permanent photograph, or, as he terms it, opto- 

 gramme, the effect of the light would have to be so prolonged or 

 so intense as to destroy the balance between the destruction of the 

 vision-purple and the power of the retinal epithelium to restore it. 

 Kiihne took a coloured rabbit and fixed its head and one of 

 its eye-balls at a distance of a metre-and-a-half from an opening 

 thirty centimetres square, in a window-shutter. The head was 

 covered for five minutes by a black cloth and then exposed for 

 three minutes to a somewhat clouded midday-sky. The head 

 was then instantly decapitated, the eye-ball which had been 

 exposed was rapidly extirpated by the aid of yellow light, then 

 opened, and instantly plunged in 5 per cent, solution of alum. 

 Two minutes after death the second eye-ball, without removal 

 from the head, was subjected to exactly the same processes as 

 the first, viz., to a similar exposure to the same object, then 

 extirpation, &c. 



On the following morning the milk-white and now toughened 

 retinje of both eyes were carefully isolated, separated from the 

 optic nerve, and turned ; they then exhibited on a beautiful rose- 

 red ground a ntarly square sharp image zuith sharply-definea 

 edges ; the image in the first eye 7vas somewhat roseate in hue and 

 less sharply defined than that in the second, which was perfectly m 

 white. The size of the images was so??iewhat greater than cm ** 

 square millimetre. 



Prof. Bunsen was amongst the witnesses of this beautiful 

 experiment. Arthur Gamgee 



1 I have repeated all the more important observations of Kiihne with the 

 eyes oi several Ranee teiiiporari(F, and with those of two rabbits, of wtiich 

 one was an albino, and can entirely confirm all his interesting facts. In 

 ordinary daylight, the purple-red colour of the frog's retina, and^ its subse- 

 quent decolorisation, may be most satisfactorily demonstrated. The use of 

 the dark chamber illuminated by sodium is, however, useful in cases where 

 the dissection of the eye has to be conducted with care. — A. G, 



I 



