[Footnote W: All told, there are hardly more than half dozen experimental investigations of the color-sense in mammals,--to speak only of these. Three of them deserve especial mention. One, the work of the American, Kinnaman,[33] on two Rhesus monkeys. Then a brief but careful piece of work by Himstedt and Nagel.[34] These two investigators were able to determine that their trained poodle could distinguish red of any tone or shade from the other colors, and from Professor Nagel I learned that later the tests were extended and the same was shown to be true concerning the blue and the green. And finally there is an investigation which hitherto has been known only from a reference which Professor Dahl,[35] the investigator, himself makes. The work is on a monkey, Cercopithecus (Chlorocebus) griseoviridis Desm. (Professor Dahl has kindly allowed me to look over the records of the experiments. He intends to publish the monograph at an early date.) All of these investigators arrive at the conclusion that the animals tested by them possess color-sense. The monkey last-mentioned shows one peculiarity: it was unable to distinguish a saturated blue from the black. It will require further tests to clear this up.] Nor did Hans possess anything like that high degree of visual acuity which had been attributed to him. He was supposed to be able to read easily at a distance small, almost illegible script, which we ourselves could decipher only with the greatest difficulty close at hand. It was also supposed that he could distinguish ten-and fifty-pfennig pieces whose faces had become worn beyond recognition for us. None of these accomplishments have stood the test. We have no reason to believe that Hans can see the objects about him more clearly than other horses, regarding whom one usually assumes that they receive only vague visual impressions. Horses do not as a rule seem to be near-sighted as is often asserted by the layman, but rather somewhat far-sighted, or if we may believe Riegel,[37] who tested some six hundred horses, they probably have normal vision. But we are told that many horses--and according to some authors all--have an innate imperfection which detracts considerably from the clarity of vision. This imperfection consists in an irregular formation of the sclerotic coat and of the lens of the eye.[38] The two organs do not have the same refraction in all parts. As a result, objective points are not imaged as points upon the retina. (Hence the name: astigmatism, i. e., "without points", for this disorder.) The retinal image of the object is not only vague, but also distorted.[X]