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(3) A lighted match held at a distance of four or five feet is sufficient to produce an effect.
(4) The light of a small gas flame enclosed in a lantern and cased to pass through a globular glass jar (12 inches in diameter) filled with a solution of ammoniacal sulphate of copper or biocromath of potash has also produced a change in the amount of the electro-motive power.
(5) The action of light on the eye of the frog is as follows: when a diffuse light is allowed to impinge on the eye of the frog, after it has arrived at a tolerably stable condition, the natural electro-motive power is in the first place increased; then diminished; during the continuation of light it is still slowly diminished to a point where it remains constant: and on the removal of light, there is a sudden increase of the electro-motive power nearly up to its original position. The alterations above referred to are variables, depending on the quality and intensity of the light employed, the position of the eyeball on the cushions and modifications in the vitality of the tissues.
(6) Similar experiments with the eye of warm-blooded animals placed on the cushions as rapidly as possible after the death of the animal, and under the same conditions, have never given us an initial positive variation as we have above detailed in the case of the frog but always a negative variation. The after indirective effect on the withdrawal of light occurs in the same way.
(7) Many experiments have been made as to the effect of light from different positions of the spectrum. . . . All these observations tend to show that the greatest effect is produced by those parts of the spectrum that appear to consciousness to be the most luminous, namely the yellow and green.
(8) Similarly, experiments were made with light of varying intensity and show that the physical effects we have observed vary in such a manner as to correspond closely with the values that would result if the well known law of Fechner was approximately true.
(9) The method followed in these enquiries is a new method in physiological research and by the employment of proper appliances, it may be greatly extended not only with regard to vision but also to the other senses.
In the last paragraph the experimenters are - characteristically of James Dewar - mapping out more virgin land to plough or, to change the metaphor, fresh worlds to conquer. In a progress report given on 5th May 1873, they recorded several improvements in method which they had made and which they followed up with a series of ten tabulated results three of which deserve mention - "(1) We have proved, using a frog, that the pigment cells of the skin in the vicinity of the cornea have nothing to do with the results obtained." (2) As to the effects produced by lights of different intensities; "If a candle is placed at a distance of one foot from the eye, and then is removed ten feet, the amount of light received by the eye is exactly one hundredth part of what it got at a distance of one foot, whereas the electro magnetic force, instead of being altered in the same proportion, is reduced by one third.
(3) It was apparent to us that these experiments would ultimately bear upon the theory of sense perception as connected with vision." Again on 2nd July, they reported on their experiments carried out by moonlight with the following results; "(1) The light from a beam of uncondensed moonlight, though of weak intensity and almost entirely free from heat rays is still sufficient to alter the electro motive power of the nerve and retina. (2) We have examined the phenomenon in the eyes of the following animals: the common newt, the goldfish, the rockling, the stickleback, the common edible crab, the lobster. The eye of the goldfish and rockling, both sluggish fishes, were found to resemble each other inasmuch as the vibrations in the electro motive force were slow - a marked contrast to those of the active and alert stickleback the eye of which was very sensitive to light. The experiments on the eyes of crustacea are of importance because they show that the action of light on the compound eye is the same as on the simple eye. (3) The action of light on the electro motive force of the living eye in cats and birds - pigeon and owl - has been observed by putting the cat or bird under the influence of chloroform. The eye of a snake was examined and its action resembled that of the frog. (4) Now we can state that the law of the variation in the electro motive force of the retina and optic nerve holds good in the following groups of the animal kingdom: - mammalia - aves - reptilla - amphibia - pisces and crustacea." With boldness - but with a legitimate boldness - they go on to say that the law of Fechner is not "as has been hitherto supposed a function of the brain alone but is really a function of the terminal... NEXT PAGE
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