94 



NATURE 



[Nov. 27, 1879 



Arsenic in Animals.— Prof. Ludwig has recently ( Wiener 

 Akad. Anz.) inquired into the distribution of arsenic in the 

 animal organism after ingestion of arsenious acid. The 

 objects he examined were the organs of suicides who had 

 poisoned themselves with arsenic, and of dogs which were 

 poisoned, some acutely, some chronically, with arsenic. In 

 all experiments it was found that the arsenic accumulated 

 most in the liver, and that in acute poisoning the kidneys 

 also contained abundant arsenic, whereas in the bones and 

 in the brain there was little of the poison. In case of 

 chronic poisoning with arsenic, where death did not ensue, 

 the poison was found to remain (after ingestion was stopped) 

 longest in the liver, being much sooner excreted from the other 

 organs. The results of this investigation are in direct opposition 

 to those obtained by Scolosuboff, who always found most arsenic 

 in the brain. 



Dioptrics of the Eye. — In the investigation of the 

 dioptric properties of the crystalline lens of the eye, physio- 

 logists have hitherto accepted an index of refraction of 

 the lens determined for only one condition of accommoda- 

 tion. It seemed desirable to Herr Matthiessen to attain 

 greater accuracy by ascertaining the dioptric properties of 

 the lens in different states of accommodation, the structure of 

 the lens as now known being fully considered. The subject is 

 discussed at length by him in PfliigtSs Atckiv (xix. p. 4S0). 

 In tabular form he presents a comparison of the positions of the 

 dioptric cardinal points for the human eye and for the eyes of 

 several lower animals, corresponding to different states of ac- 

 commodation, infinite distance 160 mm. and 100 mm. A com- 

 prehensive list of works on the dioptrics of the lens and the eye 

 generally is added to Herr Matthiessen's paper. 



EXPERIMENTAL DETERMINATION OF THE 



VELOCITY OF LIGHT* 

 T ET s, Fig 1, be a slit through which light passes, falling 

 ■" on R, a mirror free to rotate about an axis at right angles 

 to the plane of the paper ; L, a lens of great focal length, upon 

 which the light falls, which is reflected from R. Let M be a 

 plane mirror, whose surface is perpendicular to the line 

 R M, passing through the centres of R, L, and M, respec- 

 tively. If L be so placed that an image of s is formed on 

 the surface of M, then, this image acting as the object, its 

 image will be formed at s, and will coincide point for point 

 with s. 



If, now, R be turned about the axis, so long as the light falls 

 on the lens, an image of the slit will still be formed on the 

 surface of the mirror, though on a different part, and as long as 

 the returning light falls on the lens, an image of this image will 

 be formed at S, notwithstanding the change of position of the 

 first image at M. This result, namely, the production of a 

 stationary image of an image in motion, is absolutely necessary 

 in this method of experiment. It was first accomplished by 

 Foucault, and in a manner differing apparently but little from the 

 foregoing. 



In this case, L, Fig. 2, served simply to form the image of 

 S, at M ; and M, the returning mirror, was spherical, the centre 

 coinciding with the axis of R. The lens, L, was placed as near as 

 possible to R. The light forming the return image lasts, in 

 this case, while the first image is sweeping over the face of 

 the mirror, m. Hence the greater the distance, R M, the larger 

 must be the mirror, in order that the same quantity of light 

 may be preserved, and its dimensions would soon become 

 inordinate. The difficulty was partly met by Foucault, by 

 using five concave reflectors instead of one ; but even then 

 the greatest distance he 'found it practicable to use was only 

 twenty meters. 



Returning to Fig. 1, suppose that R is in the principal focus of 

 the lens, l ; then if the plane mirror, M, have the same diameter 

 as the lens, the first or moving image will remain upon M as 

 long as the axis of the pencil of light remains on the Jens, 

 and this will be the case no matter what the distance may be. " 



When the rotation of the mirror, R, becomes sufficiently 

 rapid, then the flashes of light which produce the second or 

 stationary image become blended, so that the image appears to 

 be continuous. Eut now it no longer coincides with the slit, but 

 is deflected in the direction of the rotation, and through twice 



the angular distance described by the mirror, during the time 

 required for light to travel twice the distance between the 

 mirrors. The displacement is measured by its arc, or, rather, 

 by its tangent. To make this as large as possible, the distance 

 between the mirrors, the radius, or distance from the revolving 

 mirror to the slit, and the speed of rotation should be made as 

 great as possible. 



The second condition conflicts with the first, for the "radius" 

 is the difference between the distances of principal focus and 

 the conjugate focus for the distant mirror. The greater the 

 "distance," therefore, the smaller will be the "radius." There 



are two ways of solving the difficulty : first, by using a lens of 

 great focal length, and, secondly, by placing the revolving 

 mirror within the principal focus of the lens. Both means were 

 employed. The focal length of the lens was 150 feet, and the 

 mirror was placed about fifteen feet within the principal focus. 

 A limit is soon reached, however, for the quantity of light 

 received diminishes very rapidly as the revolving mirror ap- 

 proaches the lens. 



The chief objection urged in reference to the experiments 

 made by Foucault is that the deflection was too small to be 

 measured with the required degree of accuracy. This de- 



flection was but a fraction of a millimeter, and when it is- 

 added that the image is always more or less indistinct on 

 account of atmospheric disturbances, as well as imperfec- 

 tions of lenses and mirrors, it may well be questioned 

 whether the results could be relied upon within less than 

 one per cent. 



In the following experiments the distance between the mirrors 

 was nearly 2,000 feet. The radius was about thirty feet, and 

 the speed of the mirror was about 256 revolutions per second. 

 The deflection exceeded 133 millimetres, being about 200 times 

 that obtained by Foucault. If it were necessary it could be 



1. 



M-- 



s* 



still further increased. This 'deflection was measured within 

 three or four hundredths of a millimeter in each observation • 

 and it is safe to say that the result, so far as it is affected 

 by this measurement, is correct to within one ten-thousandth 

 part. 



The site selected for the experiments was a clear, almost level 

 stretch along the north sea-wall of the Naval Academy. A 

 frame building was erected at the western end of the line, a plan 

 of which is represented in Fig. 3. 



The building was forty-five feet long and fourteen feet wide, 

 and raised so that the line of light was about eleven feet above 



