the rays of light, to see the bending of the rays at each refracting surface and to follow them as a beautiful cone to a focal point. The water in front of the crystalline lens represents the aqueous humor, and that behind the vitreous, both of these in the normal eye having the same refractive index as water. The artificial eye is best used in a partially darkened room. The object viewed is represented by a lamp, preferably an incandescent electric or Welsbach. A cross or other figure placed before it is an advantage. When two objects are needed at one time, one near, the other farther away, a candle represents the second. For experiments in which the object is moved, candles or small kerosense lamps are more satisfactory than the Welsbach. The following phenomena were demonstrated by each student in my laboratory: 1. Images. The inverted, real image on the retina. The comparative size of image when object is near and far. Comparative movement of object and image. 2. Accommodation. The change in the crystalline lens necessary to make a sharp image when the object is brought nearer. The "near" and "far" points of accommodation. Blurring of image when object is too near. The blurred image of a distant object when the eye is accommodated for a near one, and vice versa. 3. Function of the Iris: Spherical Aberration. The comparative sharpness and brightness of images of near or far objects when using large or small diaphragms, or no diaphragm at all. A ring diaphragm is provided by which the light may be shut out from the center of the lens and allowed to pass through the outer portion. The change of focus thus effected and "circles of diffusion" can be demonstrated. 4. Refraction. The bending of the rays at the cornea and at the lens. The convergence of the rays to a focus and divergence after passing it. The comparative refractive power of the. two lenses. 5. Scheiner's Experiment. A special diaphragm with two holes is provided for this experiment, and all details of it can be illustrated. 6. Far Sight. The eye is shortened by moving the retina forward. A clear image of a distant object can be obtained, but not of a near one. The eye is provided with a lens holder in front of the cornea, and proper lenses (“trial-case lenses") to correct this and other anomalies of refraction are supplied. 7. Near Sight is illustrated and corrected in similar manner. 8. Astigmatism. A special asymmetrical "cornea" is provided, which may be substituted for the "normal cornea." The cone of rays as seen from the side will now be found to come to a different focus from that seen from above. This is beautifully demonstrated. To focus the upright of a cross the "retina" must be placed (according to the axis of astigmatism) either behind or in front of the point at which the horizontal bar is focused. Assuming that either position of the "retina" is the normal, lenses are provided for the corresponding corrections; and the lens holder is graduated for the determination of the angle at which the axis of the cylinder must be placed. Astigmatism combined with myopia or hypermetropia may be illustrated and corrected. 9. Vision without Lens. The condition when the lens, as in case of cataract, has been removed. Improvement of vision by a very small diaphragm (pin-hole image) and by use of lenses. 10. Purkinje Sanson Images. The images formed by reflection from the surfaces of the lens and cornea. The effect on these of accommodation. II. The Function of the Cornea. It may be shown that the cornea alone can furnish an image by placing the "retina" far back. A further illustration is made as follows: A glass plate with a rubber washer is held against the mounting of the "cornea" in front; and the space between it and the "cornea" is filled with water. The "cornea" is thus thrown out of function. The image is blurred, the focus being back of the "retina." This illustrates the imperfection of vision under water. By using two "eyes" side by side, many of the phenomena of binocular vision can be demonstrated. The retina can be marked off in squares with pencil or India ink, and the theory of identical points can be illustrated. Double vision from crossed eyes is easily made clear to the student. These experiments were omitted in my laboratory only from lack of time. Doubtless other experiments could be devised. I can only add that my students have been so much pleased with the "eyes" that many of them have put in extra time studying the phenomena which they illustrate. A NEW MACHINE FOR ILLUSTRATING THE LAWS OF UNIFORMLY ACCELERATED MOTION. BY W. H. HAWKES. Instructor in Physics, Ann Arbor (Mich.) High School. It is hardly necessary to recount to teachers of physics the difficulties encountered in an attempt to illustrate and demonstrate the laws of uniformly accelerated motion, and to do it so that the results will be at all quantitative in character. These difficulties. are too apparent by experience to need discussion. We shall not take time nor space to review the many devices and methods, already too well known by their aggravating futility, employed to overcome these difficulties, but rather shall present a new and effective device for solving this perplexing problem and demonstrating the laws of falling bodies with a degree of accuracy that compares favorably with that of other standard quantitative problems in experimental physics. The value of the results obtained by any method in solving a physical problem depends in a large measure upon the extent to which we are able to eliminate sources of error. In the problem of uniformly accelerated motion the two great sources of error with which the experimenter must contend are friction of the moving parts and the personal equation. Friction may be reduced by the use of very delicate bearings to a quantity that will be hardly apparent in results. The personal equation is necessarily large, and is not so easily disposed of, even in cases where the experimenter has trained senses and good judgment; but the most trained ear or eye is not sensitive to sound or sight beyond the sixteenth of a second; hence the space traversed by a body (which is considerable at its greatest velocity) during this time. is enough to render the observed value doubtful and uncertain. The use of the inclined plane involves the errors arising from the observer's estimates of the coincidence of two dissimilar sense perceptions; either the click of the clock and the crack of the rolling ball, as it strikes the ruler held across the plane, or the position of the moving body at the instant the click of the clock is heard. In either case the demand upon the untrained perceptions of the high-school student is too great, and the consequent errors of observation render the results of little value. The old form of Atwood's machine involves nearly all the sources of error manifest in the inclined plane, so that only approximate results are obtained from its use, while the large amount of time consumed in securing sufficient data is in itself a fatal objection to its employment in laboratory classes enrolling many students. For these reasons this useful and instructive experiment has, in many laboratories, been supplanted by problems of lower importance, because they require less experimental skill and furnish more accurate results. An attempt has been made in the instrument here presented and herein described to secure results that will be not only independent of any error in judgment and sense perception, but will also render it possible to obtain a continuous record of distances traversed during corresponding intervals of time. This instrument may be called a self-registering Atwood's machine. Its essential parts (Fig. 1) are a marking device, M, controlled by a pendulum, P; a wheel, II', hung on delicate bearings (jewel) carrying upon its rim a belt of tissue paper ribbon, T, upon which the record is to be made, and to which are attached the counterpoised masses, C C, and overweight, O. The pendulum is first adjusted by leveling screws in the base of the instrument so that the needle point at its lowest end stands when at rest exactly in the center of the mercury globule, H, through which it passes on closing the circuit, P C, controlling the recorder, M. After the recorder has been adjusted, a switch, S, connects |