January 9, 1896] 



NATURE 



^1^1 



Chambers's we notice one descriptive of the Loofah, or Luffa, 

 \,y Prof. Carmody. 



We have received, in addition to the magazines and reviews 

 named in the foregoing, the Humanitarian, Sunday Magazine, 

 National, English Illustrated, and Longman's, but the articles 

 in them do not call for particular comment in these columns. 



RECENT PROGRESS IN OPTICS.^ 

 npHE reviewer who aspires to give an account of recent pro- 

 -'■ gress in any department of science, is met at the outset by 

 two causes for embarrassment. What beginning shall be selected 

 for developments called recent? What developments shall be 

 selected for discussion from the mass of investigations to which 

 iiis attention has been called ? So rapidly is the army of workers 

 increasing, and so numerous are the journals in which their work 

 is recorded, that the effort to keep up with even half of them is 

 hopeless; or, to borrow a simile employed by the late Prof. 

 Huxley, "we are in the case of Tarpeia, who opened the gates 

 of the Roman citadel to the Sabines, and was crushed under the 

 weight of the reward bestowed upon her." 



I have selected a single branch of physics, but one which can 

 scarcely be treated rigorously as single. From the physical 

 standpoint optics includes those phenomena which are presented 

 by ether vibrations within such narrow limits of wave-length as 

 -can affect the sense of sight. But these waves can scarcely be 

 studied except in connection with those of shorter and of longer 

 period. Whatever may be the instruments employed, the last 

 one of the series through which information is carried to the 

 brain is the eye. The physicist may fall into error by faulty use 

 •of his mathematics ; but faulty use of the senses is a danger at least 

 equally frequent. Physiological optics has of late become trans- 

 ferred in krge measure to the domain of the psychologist ; but 

 he in turn has adopted many of the instruments, as well as the 

 methods, of the physicist. The two cannot afford to part com- 

 pany. If I feel particularly friendly to the psychologist, more 

 so than can be accounted for by devotion to pure physics, it may 

 be fair to plead the influence of old association. If I am known 

 at all in the scientific world, the introduction was accomplished 

 through the medium of physiological optics. But, with the 

 limitations imposed, it is not possible even to do justice to all 

 who have done good work in optics. If prominence is assigned to 

 the work of Americans, it is not necessary to emphasise that this 

 Association is made up of Americans ; but, with full recognition 

 of the greater spread of devotion to pure science in Europe, of 

 the extreme utilitarian spirit that causes the value of nearly every 

 piece of work in America to be measured in dollars, we are still 

 able to present work that has challenged the admiration of 

 Europe, that has brought European medals to American hands, 

 that has Ijeen done with absolute disregard of monetary stan- 

 <lards ; work has been recognised, even more in Europe than in 

 America, as jjroducing definite and important additions to the 

 sum of human knowledge. 



In drawing attention to some of this work it will be a pleasant 

 ■duty to recognise also some that has been done beyond the 

 Atlantic — to rememter that science is cosmopolitan. The 

 starting-point is necessarily arbitrary, for an investigation may 

 last many years and yet be incomplete. To note recent 

 45rogress, it may be important to recall what is no longer recent. 



Light Wavks as Standards ov Lencth. 

 Vou are therefore invited to recall the subject of an address to 

 ^vhich we listened in this section at the Cleveland meeting in 

 1S88, when Michelson presented his " Plea for Light Waves." 

 In this he described the interferential comparer, an instrument 

 <leveloped from the refractometer of Jamin and Mascart, and 

 <liscus.sed various problems which seemed capable of solution 

 by its use. In conjunction with Morley he had already used it 

 in an inquiry as to the relative motion of the earth and the 

 luminiferous ether (American Journal of Science, May 1886, 

 P- 377) > and these two physicists together worked out an 

 elaborate series of preliminary experiments {ibid., December 

 1877, p. 427) with a view to the standardising of a metric 

 unit of length in terms of the wave-length of sodium 

 light. By use of a Rowland diffraction grating. Bell had 

 <letermined the sodium wave-length with an error estimated to 



1 Address delivered by Prof. W. LeConte Stevens before the Section of 

 Physics of the American Association for the Advancement of Science, at the 

 Springfield meeting, August 1895. 



be not in excess of one part in two hundred thousand 

 (American Journal of Science, March 1887, p. 167). Could 

 this degree of accuracy be surpassed? If so, it must be 

 not so much Ijy increased care in measurement as by 

 increase of delicacy in the means employed. The principle 

 applied in the use of the interferential comparer is simple 

 enough ; the mode of application cannot be clearly indicated 

 without a diagram, but probably all physicists have seen this 

 diagram, for it was first brought out eight years ago (ibid., 

 Deceml>er 1887, p. 427). By interference of beams of light, 

 reflected and transmitted by a plate of plane parallel optical 

 glass, and then reflected back by two mirrors appropriately 

 placed, fringes are caught in an observing telescope. One 

 of the mirrors is movable in front of a micrometer 

 screw, whose motion causes these fringes to move across the 

 telescopic field. If the light be absolutely homogeneous, the 

 determmation consists in measurement of the distance through 

 which the movable mirror is imshed parallel to itself and the 

 counting of the number of fringes which pass a given point in 

 the field of view. According to the theory of interference the 

 difference of path between the distances from one face of the 

 plate to the two mirrors should be small ; beyond a certain limit 

 interference phenomena vanish, and this limit is smaller in 

 proportion as the light is more complex. In the case of 

 approximately homogeneous light there are jieriodic variations 

 of distinctness in the fringes. For example, assume sodium 

 light, which in the spectroscope is manifested as a pair of yellow 

 lines near together. In the refractometer there are two sets of 

 interference fringes, one due to each of the two slightly different 

 wave-lengths. When the difference of path is very small, or 

 nearly the same for both of these radiation systems, the fringes 

 comcide. The wave-length for one is about one-thousandth less 

 than that for the other. If the difference of path is about five 

 hundred waves, the maximum of brightness for one system falls 

 on a minimum of brightness for the other, and the fringes 

 become faint. They become again bright when the difference of 

 path reaches a thousand wave-lengths. The case is entirely 

 similar to the familiar production of beats by a pair of slightly 

 mistuned forks. 



The method of interference thus furnishes through optical 

 beats a means of detecting radiation differences too minute for 

 resolution by ordinary spectroscopic methods. Spectrum lines 

 are found to be double or multiple when all other means of 

 resolving them fail ; and the difficulty of attaining truly homo- 

 geneous light is far greater than was a few years ago supposed. 

 By the new method it becomes possible to map out the relative 

 intensities of the components of a multiple line, their distance 

 apart, and even the variations of intensity within what has for 

 convenience been called a single component. Each of the two 

 sodium lines is itself a double whose components are separated by 

 an interval about one-hundredth of that between the long-known 

 main components ; and an interval yet less than one-fifth of this 

 has been detected between some of the components of the 

 green line of mercury. Indeed Michelson deems it quite possible 

 to detect a variation of wave-length corresponding to as little as 

 one ten-thousandth of the interval between the two main sodium 

 Vmes {Astronomy and Astrophysics, p. 100, February 1894.) 



This new-found complexity of radiation, previously thought to 

 be approximately if not quite simple, proved to be a temporary- 

 barrier to the accomplishment of the plan of using a light-wave 

 as a standard of length. It necessitated careful study of all 

 those chemical elements which give bright lines that had been 

 supposed to be simple. The red line of cadmium has been 

 found the simplest of all those yet examined. The vapour in a 

 rarefied state is held in a vacuum tube through which the electric 

 spark is passed, and under this condition the difference of path 

 for the interfering beams in the refractometer may be a number 

 of centimetres. A short intermediate standard, furnished with 

 a mirror at each end. is now introduced into the comparer, and 

 moved by means of the micrometer screw. Its length is thus 

 measured in terms of the cadmium wave-length. A series of 

 intermediate standards, of which the second is double the first, 

 the third double the second, &c., are thus compared, and finally 

 in this way the value of the metre is reached. 



The feasibility of this ingenious method having been made 

 apparent, Michelson was honoured with an invitation from the 

 International Bureau of Weights and Measures to carry out the 

 measurement at the observatory near Paris, with the collaboration 

 of the director M. Benoit. After many months of labour, results 

 of extraordinary accuracy were attained. For the red line of 



NO. 1367, VOL. 53] 



