OCR Rendition - approximate

. MARCH 7, 1907] NATURE 451 The abnormality of the distribution of the estimates now becomes manifest, and is of this kind. The competitors may be imagined to have erred normally in the first instance, and then to have magnified all errors that were negative and to have minified all those that were positive. The lower half of the " observed " curve agrees for a large part of its range with a normal curve having the p.e.=45, and the upper half with one having.its p.e.=29. I have not sufficient knowledge of the mental methods followed by those who judge weights to offer a useful opinion as to the cause of this curious anomaly. It is partly a psychological question, in answering which the various psychophysical investigations of Fechner and others would have to be taken into account. Also the anomaly may be partly dud to the use of a small variety of different methods, or formula=_, -so that the estimates are not homogeneous in that respect. It appears then, in this particular instance, that the vex populi is correct to within r per cent. of the real value, and that the individual estimates are abnormally distributed in such a way that it is an equal chance whether one of them, selected at random, falls within or without the limits of -3-7 per cent. and +2-4 per cent. of their middlemost value. This result is, I think, more creditable to the trustworthiness of a democratic judgment than might have been expected. The authorities of the more important cattle shows might do service to statistics if they made a practice of preserving the sets of cards of this description, that they may obtain on future occasions, and loaned them under proper restrictions, as these have been, for statistical discussion. The fact of the cards being numbered makes it possible to ascertain whether any given set is complete. i FRANCIS GALTON. T THE WORK OF THE OPTICAL SOCIETY.' HOUGH it is perhaps seldom that the Transactions of the Optical Society contain much record of original research, yet they often furnish matter of- considerable value to the practical optician, and will usually be found to contain more than one paper of importance to the student of optics. The first paper in the present volume will have interest for many readers. It is a short and Clear statement, by one well qualified to judge, Mr. W. A. Dixey, of the case for the use of periscopic lenses in spectacles. A periscopic lens, as defined by Mr. Dixey, is one through which its wearer can look obliquely as well as directly without his vision being impaired by radial astigmatism. The result is attained either by deepening the curves of the lens so as to produce an , approximation to a sphere the centre of which coincides with the centre of rotation of the eye, or by the use of toric lenses. A careful reading of Mr. Dixey's paper would probably lead many wearers of spectacles to pay another visit to the optician. The paper by Mr. Theodore Brown, on direct stereoscopic projection, is of special interest. It describes an ingenious device for obtaining stereoscopic effects in pictures projected on a screen, of which probably more will be heard. The argument is that in binocular vision the stereoscopic effect is due to the fact that the two images of the same object in the two eyes are not formed in similar positions on the retina, and that it should be possible to produce stereoscopic effects even when one eye only is used if by some means two simultaneous pictures can be formed on the retina in appropriate positions. Owing to the permanence of retinal impressions, this can be effected by throwing on the screen with rapid alternation the two stereoscopic pictures in somewhat displaced positions. The displacement is produced by giving a motion to the projection camera, afnd the stereoscopic can be combined with the "` bioscopic " effect 'by the use of . a kinematograph ittechanism. It is clear from the paper and the discussion that further perfection of detail is necessary to produce completely satisfactory results, but there seems no reason vvhv the difficulties should not be overcome.- There is, t transactions of the Optical Society, London, Session t9o4-5. PP. 93. Puce sot. NO. 1949, VOL, 7 5] however, some reason for suggesting that perhaps a onee_ved spectator would be the most appreciative. In " A Method of Testing Prisms," Mr. S. D. Chalmers, the head of the optical department at the Northampton Institute, suggests some useful applications of the method of auto-collimation for the determination of the angles of prisms to the highest possible accuracy. We would direct attention especially to the procedure suggested for the measurement of one of the angles of a 60° prism ABC. Rays entering perpendicular to the face Bc are totally reflected at 30' from c.a or AB, and reflected normally at AB or CA, emerging again approximately perpendicular to BC. From the separation of the incident and emergent rays the error in the angle A can be determined. Only one reflection takes place normally at a glass-air surface, and there is, therefore, no difficulty in seeing the image. Simultaneous observation of the direct reflection from rc aids in setting the prism. A similar method can readily be applied in other instances, and the figures in the paper suggest at once the procedure in the cases which occur most frequently. The lack of parallelism in plane parallel glass can also be tested in this manner. Rigidity is, of course, essential in the apparatus to be employed for the observations, and Mr. Chalmers has obviously given some attention to details in the construction of a suitable autocollimator; drawings cr diagrams of this would have added to the value and interest of the paper. Mr. L. WV. Phillips, student member, in a paper on the measurement of absorption in tinted glasses, describes some observations on the absorption of light by coloured glasses, such as are used for spectacles, a " flicker " photometer being employed for the measurements. The method does not admit- of great accuracy, but is no doubt useful within ceitain limits for rough work of the kind suggested. It raises, however, some vexed questions as to the photometry of coloured lights, and some points of interest in relation to the " flicker " instrument were brought up in the discussion by Dr. Garnett and others. A presidential address by Prof. Silvanus P. Thompson, on the early literature of optics, demands no more than passing mention here, great though its interest must have been to those who had the pleasure of listening to it. The volume closes with a contribution by Mr. A. T. Bull, entitled " Some Notes on the Nature of Vision." The paper, being essentially of the nature of notes, passes rapidly over many points of interest in connection with the mechanism of vision and the molecular and other phenomena accompanying it. Various topics are thus touched on, from the accuracy of the photometric results obtained with rotating sectors to the difficulties yet requiring explanation on the Darwinian theory in regard to the process of evolution of vision. The notes are suggestive, but would make more interesting reading if less loosely put together. We would venture to suggest, in conclusion, that the editing of the volume leaves something to be desired. It may be possible to interpret such phrases as " beep lenses on the Willaston principle," and to escape being deceived by the " dissimulation " of a photochemical body, but the fact that Lewis Carroll was a mathematician does not sufficiently justify such imitation of his playful extravagances in a scientific journal ; and the volume is marred by many such misprints. THE COMPRESSIBILITY OF CRYSTALLINE ROCKS.' T HE latter-day revival of interest in geological physics has led to a keen demand for experimental data, the absence of which has hitherto rendered futile most speculation in this domain. Our almost complete ignorance of the simplest physical constants of rocks and the rockforming minerals is easy to account for. The kind of investigation required is both difficult and laborious, calling for skill and practice as well as the appliances of a well-equipped physical laboratory ; and the geologist may lack either the capacity or the opportunity for such re r " An Investigation into the Elastic Conctsntc of Rocks, more Especially with Reference to CuFic Compressibility." By Prof. Frank D. .Adams and Prof. Ernest G. Coker, Pp. 69. (Washington, D.C. : Carnegie Institution, r9o6.) CIibPDF - www.fastio.com