April 16, 1903] 



NA TURE 



559 



Frequently he examined the interior of the box when he 

 had opened it, but food was never placed inside it. One 

 evening, after the trick had been shown to a number of 

 friends in order that the dog's almost ridiculous familiarity 

 with it might be noted, Peter was sent to bed without his 

 supper. He is fed but once a day. Next morning a hot 

 grilled bone was placed in the box. The box was placed in 

 a small yard surrounded by the house. The " boot-room " 

 opens into the yard on one side, and into a passage on the 

 other. After the dog had had a run in the garden the 

 passage door into the boot-room was opened. We were 

 watching the yard from an upper window. Two minutes 

 after entering the boot-room Peter smelled the bone, ran 

 through into the yard, and approached the box. When he 

 saw the latch he ducked his head as if intending to lift it, 

 but desisted. He then sniffed excitedly at the box and 

 pushed it with his nose. -He returned to the boot-room. 

 After a few minutes he came out again into the yard and 

 sniffed in the same way at the box. Twice he pushed the 

 latch from behind, but did not put his head beneath it. 

 After a while he returned to the boot-room and showed no 

 signs of revisiting the box. He was then taken for a 

 twelve-mile run in the country. As he seemed to be tired 

 when he reached home, he was left for half an hour in the 

 boot-room to rest. After a run in the garden, he was re- 

 admitted to the boot-room, with the yard-door open. Un- 

 luckily the wind blew the door to before Peter had gone into 

 the yard. After we had watched for some time my son went 

 down to see what had happened — opened the door and 

 pushed the dog through it, backwards. He went straight 

 to the box, lifted the latch in the most business like way, 

 and took out the bone. 



The experiment was repeated a fortnight later with 

 identical results. The dog ran into the yard, sniffed at the 

 box, pushed it with his nose, was very eager to get the 

 meat, but, this time, he showed no sign of remembering 

 the way to open the box. He returned a second time, and 

 then desisted altogether. During the morning the dog re- 

 mained about the house. He constantly asked to be ad- 

 mitted into the boot-room, and showed in the clearest 

 manner that he remembered that the grilled bone was to be 

 found that way. At twelve o'clock the door was opened for 

 him. He went straight through into the yard, opened the 

 box, and took out the bone, which he attacked without any 

 sign of doubting his legal right to its possession. It may be 

 noticed that he is frequently fed in this yard. 



In this experiment the dog knew two things. He knew 

 how to open the box. Indeed, the sight of the latch was 

 so strongly associated in the dog's mind with the action of 

 lifting it that it is surprising that the usual, almost 

 mechanical, response to sensation did not occur. Had he 

 lifted the latch it would not necessarily have implied that he 

 did it with the object of securing the food. He knew that 

 the box contained meat. Eager as he was to secure the 

 meat, he did not reason " The way to secure the meat is to 

 lift the latch." I have described the experiment in detail, 

 because all details are, as it appears to me, of great import- 

 ance. It is to be noted that the opening of the box was 

 associated in the dog's mind with the approbation of a 

 human being. Great care was taken that no person should 

 be present when the dog found the box. The sight of the 

 box was strongly suggestive to the dog's mind of the action 

 of opening it. With a view to diminishing the urgency of 

 this sensori-motor association, a piece of hot meat with a 

 strong " brown smell " was placed in the box. Its rich 

 scent distracted his attention from the latch. When the dog 

 was readmitted to the yard later in the morning, he was 

 aware that the box was in the yard, and he went straight 

 from a person to the box. By this time the bone was cold, 

 and its scent less striking. It is impossible to repeat the 

 experiment upon Peter, because now, when he opens the 

 box, he invariably searches for food inside it. But I should 

 be grateful to any of your readers who would repeat this 

 experiment, taking great care (i) that the opening of the 

 box is not associated in the dog's mind with finding food 

 inside it, and (2) that, when the dog finds the box containing 

 food, he is quite alone. I need hardly add that I shall be 

 still more grateful to anyone who will suggest to me another 

 test of the same kind. Alex. Hill. 



Downing College Lodge, Cambridge. 

 NO. 1746, VOL. 6/] 



Spherical Aberration of the Eye. 



An account of the recognised methods of investigating 

 the spherical aberration of the eye is given by Tscherning, 

 " Rapports presented au Congres international de Physique 

 r^uni a Paris en 1900," tome iii., pp. 551-557. These 

 methods for the most part require special experimental 

 appliances, and for some to succeed it is necessary to resort 

 to cocaine or homatropine injections in order to increase 

 the size of the pupil. The following method, which requires 

 no special apparatus or preparation, appears to have escaped 

 observation, and may therefore be worth describing. Place 

 a piece of white paper, on which a broad black band has 

 been ruled horizontally, just beyond the shortest distance 

 of distinct vision from the eye, and while looking at the 

 upper edge of the black band, cover the pupil progressively 

 from below by means of a card with its upper edge hori- 

 zontal, placed as near as possible to the eye. At the moment 

 when the pupil is all but completely covered, the edge of 

 the black band will be seen to suffer a depression, its 

 original position being regained on uncovering the pupil. 

 On raising and lowering the card at a rate of once or twice 

 a second, this displacement is very marked. The best 

 success is obtained in a fairly dim light, when the pupil is 

 expanded ; care must be taken to keep the eye carefully 

 focused on the edge of the black band, or an exaggerated 

 displacement, due to relaxation of the accommodation of 

 the eye, may result. The above experiment shows that, 

 when accommodated for near vision, the optical system of 

 the eye is over-corrected for spherical aberration, the rays 

 transmitted near the edge of the pupil being insufficiently 

 deviated. To prove this, let us suppose the edge of the 

 black band to be situated on a continuation of the optic axis 

 of the eye. Then, provided the accommodation of the eye is 

 correct, the rays traversing the middle of the pupil will form 

 an image of the edge of the black band at that point of the 

 retina which is cut by the optic axis. If the rays trans- 

 mitted through the upper peripheral portion of the pupil 

 are insufficiently deviated, they will cut the retina at a 

 point above the true image, and owing to the mental 

 inversion of retinal images, an image apparently below 

 the true image will be observed. On covering up the 

 pupil from below, the true image is obscured, and that 

 formed by the rays traversing the upper edge of the pupil 

 is alone seen. 



On repeating the above experiment, when the eye is fixed 

 on a distant object, the image of the latter will apparently 

 rise, showing that it really sinks, as the pupil is covered 

 from below. This proves that, when at rest, the optical 

 system of the eye is under-corrected for spherical aberration, 

 thus resembling an ordinary lens. 



If an image of a gas flame is formed on a white card by 

 means of a lens of three or four inches focus, the depression 

 of the image on the card, as the lens is progressively covered 

 from below, can easily be observed. EdwiH Edser. 



April 2. 



The Name Solenopsis. 



It appears from your issue of March 19 (p. 480) that Dr. 

 Wheelton Hind was to read a paper before the Geological 

 Society on March 25, on a new species of Solenopsis. We 

 have here an illustration of the extraordinary persistence 

 of an untenable name. The name Solenopsis was bestowed 

 by Westwood in 1841 (An. Mag. Nat. Hist., vi. p. 86) on 

 a very common and well-known genus of ants. In 1844 

 McCoy gave the same name to the genus treated of by Dr. 

 Hind, which consists of Mollusca occurring fossil in the 

 Carboniferous rocks. This Molluscan genus (which was 

 made the type of a family Solenopsida; by Neumayr) cannot 

 possibly retain the name it bears, and it may be called 

 Solenomorpha. 



I observe that recently Reiffen has proposed the name 

 Ludwigia for a genus of echinoderms. The same name 

 was bestowed by Pic in 1S93 on a group of beetles. More 

 strange is Distant 's recent proposal of Melania for a genus 

 of Coreid bugs, this being the name of one of the best- 

 known of Molluscan genera ! 



T. D. A. Cockerell. 



E. Las Vegas, N.M., U.S.A., April 2. 



