158 PROFESSOR FRANZ BOLL. 



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 

 observation of Boll, have not only corrected many errors 

 which that observer had committed, but have led to the dis- 

 covery 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.i 



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 15th, in which he announces the startling con- 

 firmation to his previous researches afforded by his having 

 been able to obtain actual images on the retina which corre- 

 sponded with objects xohich had been looked at during life (!) 



The discoveries of Boll and Kiihne must, as the latter re- 

 marks, have led to the thought that after all there might be 

 some truth in the stories which we all have heard of images 

 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, optogramnie, 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 eyeballs at a distance of a metre and a half from an 

 opening thirty centimetres square, in a window-shutter. The 



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

 eyes of several RitKe temporanae, and with those of two rabbits, of which 

 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. 



