surface. The eye sees it move in and remembers that it is there, but if we merely the light for a mon and no longer sees it. Another example is change-in-contrast detection. If there is an edge moving in or out there are pulses, but if the thing stands still there are no pulses at all Then there is a dimming detector If the light intensity is going down it creates pulses, but if it stays down or stays up, the impulse stops; it only works while the light is dimming Then, finally, there are a few fibers which are dark detectors-a most amazing thing-they fire all the time! If we increase the light, they fire less rapidly, but all the time. If we decrease the light, they fire more rapidly, all the time. In the dark they fire like mad, perpetually saying, It is dark! It is dark! It is dark Now these responses seem to be rather complicated to classify, and we might wonder whether perhaps the experiments are being misinterpreted. But it is very interesting that these same classes are very clearly separated in the anatomy of the frog! By other measurements, after these responses had been classified(afterwards, that is what is important about this), it was discovered that the speed of the signals on the different fibers was not the same, so here was another, independent way to of a fiber we have found! Another interesting question is from how big an area is one particular fiber making its calculations? The answer is different for the different classes Figure 36-14 shows the surface of the so-called tectum of a frog, where the nerves come into the brain from the optic nerve. All the nerve fibers coming in from the optic nerve make connections in various layers of the tectum. This layered structure is analogous to the retina; that is partly why we know that the brain and etina are very similar. Now, by taking an electrode and moving it down in suc ession through the layers, we can find out which kinds of optic nerves end where, K. and the beautiful and wonderful result is that the different kinds of fibers end in lifferent layers! The first ones end in number 1 type, the second in number 2 the threes and fives end in the same place, and deepest of all is number four (What a coincidence, they got the numbers almost in the right order! No, that is hy they numbered them that way, the first paper had the numbers in a different order!) We may briefly summarize what we have just learned this way: There are three pigments, presumably. There may be many different kinds of receptor cells con- taining the three Its in different proportions connections which may permit additions and subtractions through addition and Fx reinforcement in the nervous system. So before we really understand color vision we will have to understand the final sensation. This subject is still an open one, but these researches with microelectrodes and so on will perhaps ultimately give us Fig. 36-14. The tectum of a frog how we see color BIBLIOGRAPHY Committee on Colorimetry, Optical Society of America, The Science of Color, Thor Mechanisms of Vision, "2nd Supplement to Journal of General Physiology, Vol. 43 No. 6, Part 2, July 1960, Rockefeller Institute Press DEROBERTIS. E, "Some Observations on the Ultrastructure and Morphogenesis HURVICH, L. M. and D. JAMESON, "Perceived Color, Induction Effects, and Opponent- Response Mechanisms, "pp 63-80 ROSENBLITH, W. A, ed, Sensory Communication, Massachusetts Institute of Tech nology Press, Cambridge, Mass., 1961 Sight, Sense of, "Encyclopaedia Britannica, Vol. 20, 1957, pp. 628-635