On the photochemistry of the retina and on visual purple / translated from the German of W. Kühne ; edited, with notes, by Michael Foster.

Kühne, W. (Willy), 1837-1900.

Date:: 1878

Credit: On the photochemistry of the retina and on visual purple / translated from the German of W. Kühne ; edited, with notes, by Michael Foster. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

100/114 (page 92)

$invariably inferior to the morning. The more intense the light and the shorter the necessary exposure, so much the better Avere the optograms. As far as the time after death is con- cerned, the following rule proved general: that the exposure must first of aU be long, then shorter, then after a while longer again. If, for example, one eye immediately after de- capitation requires five minutes' exposure, the other eye taken immediately afterwards will give in three minutes an image of equal sharpness on a ground of equally deep purple ; while under the same light the eye thirty minutes after decapitation requires from five to seven minutes, and in an hour or an hour and a half a still longer time to bring out an optogram. I am not prepared to say that eyes which have remained in the head an hour or more after decapitation will no longer give satis- factory optograms; indeed, the limit for obtaining a good image seems to be in rabbits from about sixty to ninety minutes, while the eyes of oxen seem to be useless after one hour. Still I have never worked with the latter otherwise than after removal from the head, which I find in the case of rabbits shortens the period during which they are available. Most probably this point depends largely upon temperature. To save others trouble and time, I may as well add, that when the severed head manifests snapping movements, and spasms develope in the ocular muscles, it is well to destroy the brain completely with a gum catheter, in order to prevent the optogram from being spoiled. The reason why it is in all cases necessary to make the exposure longer immediately after death than when the experiment is conducted somewhat later, seems to me to lie m the maintenance of the power of regeneration, which ap- parently continues the longer the less the eye is touched, and therefore lasts longer in the head than in the isolated bulb, and is shortest in the opened eye. The necessity on the other hand which is felt of increasing the time of exposure again at a still later period, I would explain by the assumption that in such cases the increasing turbidity of the various media of the eye, as well as of the retina, allows chiefly the less refractive rays to pass through, which act much more slowly on the purple. There seems to be no other explanation pos-$