Ficniu-ARY 1, 1895.] 



SCIENCE. 



115 



Dr. Curtis proposes to edit, iiiul to have 

 published, a translation into English of the 

 entire treatise, the Greek portion to be 

 translated by himself, and the inedited 

 Arabic portion l\v a eollaborator not yet 

 named. 



This English translation will be the first 

 complete edition of the ' epoch-making ' 

 Galenic work in ((uestion published in any 

 language since the invention of printing. 

 The Normal Defect of Vision in the Fovea. 



Mrs. C. L. Franklin. (Introduced by 



H. P. BowDiTcn.) 



Konig's announcement, in May, ISOi, 

 that the relative absorption by the visual 

 purple of the ditl'erent portions of the spec- 

 trum is in very close comcidence with the 

 relative brightness of the dift"erent portions 

 of the spectrum, (1) for the totally color- 

 blind, and (2) for the normal eye for faint 

 light after adaptation (with the obvious in- 

 ference therefrom that the vision of the to- 

 tally color-blind and that of the normal eye 

 in a faint light was conditioned upon the 

 presence of the visiuil purple in the retina), 

 made necessary some assumption to take 

 account of the fact that in the fovea, which 

 is the portion of the retina where vision is 

 most acute, no visual purple has hitherto 

 been found. Two assumptions were possi- 

 ble, either that the cones (and hence the 

 fovea) do contain visual purple, but that it 

 is here of such an e.\tremely decomposable 

 character that it can never, no matter what 

 precautions are used, be detected objec- 

 tively ; or, that vision does actually not 

 take i)lace in the fovea under the above cir- 

 cumstances (that is, for the totally color- 

 blind and for the normal eye at sucli inten- 

 sities as are visible only after adaptation ). 

 As I had already made the prediction tliat 

 total color-l)lindncss consists in a non-de- 

 velopment of the eonen of the retina (Zfsch. 

 f. J'ni/rh. n. Phijs. der Sinnenorgane, Bd. IV.) 

 and also that the adaptation which renders 

 vision possible after twenty minutes in a 



faint light is conditioned by the growth of 

 the visual purple (.»//«</, N. S., III., p. lO:?), 

 both predictions being naturally suggested 

 t)y my theory of light-sensation, I was most 

 anxious to put the latter assumption to the 

 test. I therefore undertook to determine, 

 in the dark rocmis of Prof. Kiinig's laboi-a- 

 tory, the threshold for light-sensation for 

 ditferent parts of the retina and for ditlerent 

 kinds of monochromatic light. 



The blindness of the fovea for faint light 

 did not at once reveal itself; the act of 

 fixation means holding the eye so that an 

 image falls on the part of the retina best 

 adaiited for seeing it, and hence it would 

 involve keeping the image out of the fovea 

 in a faint light, if the fovea were really 

 blind in a faint light. But after the total 

 disa])i)earance of the .small bright object 

 looked at had several times occurred by ac- 

 cident, it became possible t<j execute the 

 motion of the eye necessary to secure it at 

 pleasure. It was then found that the simple 

 devices of presenting a group of small 

 bright objects to the eye of the ol)server 

 was sufficient to demonstrate the ' nomial 

 night-blindness of the fovea " (as it may best 

 be called) without any ditlicnlty ; one or 

 the other of them is sure to fall into the dark 

 hole of the fovea by accident. It was only 

 by means of this arrangement of a number 

 of small bright spots that the total blindness 

 of the totally color-blind boy in the fovea 

 could lie detected ; he had, of cour.se, 

 learned nut to u.se his fovea in fixation. 

 Professor Konig then proceeded to demon- 

 strate the total blindness in the fovea of the 

 normal eye to blue of about 470.-'' 



[These experiments upon the normal 

 eye were exhibited.] — It was shown that 

 Konig's i)roof that the pigment-epithelium 



*Professor v. Kries is said h\ Protc.'isoi'iiad toliavu 

 shown tliatthc experiments in (jucation do not estab- 

 lisli the Wk( -blindness ot tlie fovea { BtriMe ilrr Xa- 

 turforKclicniUn (Itwlhtlmft zu Freihurg, IX., 'i. S. 61 ). 

 I have not vet lia<l nceess to this criticism. 



