A quality of the nervous element of the eye, that of inverting all images painted on the retina, was the cause of the inversion illusion described in the May 25 issue of the SCIENTIFIC AMERICAN; the cause of the illusion described here is, on the contrary, a serious though ever-present defect of the normal eye. With a single stroke of your penknife make a slit nearly one inch long in the middle of a large piece of dark-shaded pasteboard. Over half the length of the slit lay a piece of blue glass, and keep it in place with some gummed paper stuck on the edges of the glass, so as to let light pass freely from the slit through the glass. Over the other half of the slit fix in a similar manner a piece of red glass. This being done, the card, seen on the opposite side, will look as shown on the following figure, B and R being the blue and red halves of the slit. Take another piece of the same pasteboard, and near a corner make two pin-holes one-eighth of an inch apart. The apparatus is now complete and ready for use. Place yourself near a source of light, lamp or window. Bring the card bearing the two pinholes in contact with one eye, the two holes lying on a horizontal line, and, through them, look at the vertical blue and red slit, B R. on the other card, this being placed at a distance of about one foot from the eye and right in the middle of the field of vision. The following figure shows the appearance of the card thus observed. Two luminous red slits and only one blue are perceived. The card bearing the slit may be inverted. The result is the same; the red goes up, the blue goes down, the red slit is duplicated, the blue is not. As everything but the color is symmetrical, right and left, up and down, on both cards, any observer will promptly reach the conclusion that the cause of the illusion lies, not in the position, size, or shape of the slits or pinholes, but in the difference in color of the slits. The inference is right. Owing to its defective achromatism, the eye does not bring red and blue rays to one and the same focus, and the little apparatus shows that this aberration is far from being an insignificant factor of the imperfection of normal sight. The following two figures show the path of the luminous pencils emitted by the blue and red slits and admitted by the two pinholes. (Distance from slit to eye has been shortened to spare space, and the refraction phenomena in the several humors has been simplified.) The blue rays emitted by the slit B are brought to a focus, t, on the retina, but the red rays are less bent than the blue; and although the distance of both blue and red slits from the eye is the same, the red pencils emitted by the slit R, and admitted by the two pinholes, strike the retina in r r, before meeting, and give thereby the impression of a double red slit. If the card bearing the slit is placed at much less than one foot from the eye, two blue slits, much closer together than the red ones, will be perceived. The blue pencils, in that case, also meet behind the retina, although not so far behind it as the red pencils under the same circumstances. The card with the slit must be large, to prevent side light from partially closing the pupil, thus increasing notably the achromatism of the eye. For a similar reason, if the two pinholes are much less than one-eighth of an inch apart, the experiment is much less remarkable; the central instead of the marginal parts of the crystalline lens are made to work, and increased achromatism is the result. No Fire Peril in Pennsylvania Terminal. Contracts have been closed for the construction of floors, interior partitions, and roof of the Pennsylvania's new Manhattan Passenger Terminal. Protection of the 400,000 passengers who will pass in and out of this station daily has absorbed the attention of the company's engineers a long time, and to avoid the possibility of fire they have selected a material, every block of which is tested by being heated red hot before being used, to be sure that it is un-burnable. Porous terra cotta was the only building material which was found able to withstand such a test, and the railroad's engineers have decided to use it in the form of hollow blocks as a covering for the gigantic steel frame of the building, and for the partitions and roof, as well as for lining the outside walls. If all the blocks used in this work should be built into a wall ten feet high, it would stretch from the station to Newark, N. J., a distance of twelve miles. The " Mauretanla Beats the Record. In spite of a fresh gale and a high following sea during her last passage, the "Mauretania" beat the best eastward time of her sister ship, the "Lusitania," by 21 min., completing the run from Sandy Hook to Daunt's lightship in 4 days, 22 hours, and 29 minutes. The average speed was 23.69 knots, and the best day's runs were 556 knots and 554 knots respectively. The Lady of Senbtes. During the excavations made under the direction of the Metropolitan Museum of Art in Egypt, a mummy now known as the "Lady of Senbtes" was discovered. It is believed that it is the oldest mummy that has been discovered so far. When the tomb was opened, Dr. Eliot-Smith, professor of anatomy at the Cairo School of Medicine, was called in, and made an interesting report. He found that the lady was of the twelfth dynasty, about 4,000 years ago. She was fifty years old at the time of her death and was slender, with a small, almost infantile face. Her eyes were large and round, and her nose well proportioned, being neither aquiline nor flat. Her teeth were remarkably well preserved. The mummy was that of a person of very high caste, as was shown by the ornaments buried with it. On the breast was a dagger in a gold-capped sheath of wood, and in the hair was a weave of gold filigree work profusely decorated with gold rosettes. Perhaps the most interesting find in this connection was the discovery of the ceremonial whip of Senbtes, which was lying beside the mummy in the coffin. This whip, the only perfect one in existence, was swung ceremonially by those high in authority. Arbitration proceedings between the Ottoman government and the Ottoman Railway Company have been commenced before Senor Maret, the ex-premier of Spain, who has been nominated umpire by the Kaiser, the former arbitrator. The chief matters in dispute, says Reuter, relate to the harbor works at Salonika, the port of Dedeagatch, and questions of customs and maritime transports. The sum involved amounts to about $16,000,000.
This article was originally published with the title "A Chromatic Illusion" in Scientific American 97, 25, 462 (December 1907)