86 



SCIENCE. 



[Vol. X. No. 237 



speaker's giving a method by wliich any one can measure the size 

 of the pupil of his own eye, all the apparatus needed being easily 

 constructed from a half-sheet of writing-paper. Mr. Brashear ex- 

 hibited a new form of comet-seeker, very conveniently arranged, 

 consisting of a Newtonian reflector of six and one-half inches clear 

 aperture, mounted with an altazimuth motion, the eye-piece being 

 placed in the horizontal axis, so that the observer has no need to 

 change his position while sweeping the sky. He also showed a 

 modification of the Merz-Young polarizing helioscope, less liable to 

 breakage than the old form ; and in a third paper he advocated the 

 adoption of standard sizes for screws and draw-tubes for astronomi- 

 cal instruments, so that the parts may be interchangeable, — an im- 

 provement whose utility seems self-evident, especially if combined 

 with the adoption of metric units for the standards. 



Professor Boss presented a list of over twelve hundred stars with 

 large proper motions, whose data, collected from various sources, 

 will, however, need to be verified before detailed publication. 



The last paper of the day had been postponed from the first place 

 on the list, and was by Prof. F. N. Willson. He presented a 

 systematized nomenclature for the roulettes, or trochoidal curves, 

 in which he recognizes the fact that the same curve may be gen- 

 erated in two ways by varying the relative ' size of the circles and 

 the position of the tracing-point. 



On Friday morning only seven papers were assigned to Section 

 A, and two of these failed to appear. Mr. Woodward's paper on a 

 method of computing the secular contraction of the earth, and 

 Professor Stone's on the perturbations of the orbit of Hyperion, 

 were of a technical character, consisting of a discussion of the dif- 

 ferential equations involved in the problems. The interest of the 

 morning, however, centred about the papers of Professors Menden- 

 hall and Webb. The former was upon the eccentricities of guess- 

 ing. The circumstances which gave rise to it were these : a num- 

 ber of persons having recorded their guess of the number of nails 

 contained in a glass carboy, the nails being of various sizes, large and 

 small, these guesses were given over to the professor for discussion. 

 The results were plotted with amount of guess and number of guess- 

 ers as abscissa and ordinate, and were found to agree pretty well 

 with the probability curve ; but the maximum of the curve did not 

 coincide with the actual number of nails, showing that the average 

 guess was considerably below the truth in this case, reasons for 

 which were easily suggested. Mr. Farquhar suggested that the 

 use of the logarithm of the number guessed for abscissa would 

 bring the two curves into closer agreement. The author rather 

 objected to his own title, and thought this was not really a case of 

 guessing (which should be entirely without the bias of any 

 reason), but rather a series of estimates. It is no doubt true, that, 

 while some of the numbers were careful estimates, others were the 

 most random guesses, made without ever seeing the carboy. The 

 guesses numbered over seven thousand, and varied from forty-three 

 to over three million, both extremes being seriously given. The 

 true number was 2,551, as ascertained later by actual count. 

 Professor Webb advocated the introduction of the idea of mass 

 into the definition and formula for the moment of inertia, defining 

 it as the summation of the mass into the square of the distance 

 from the axis, or the mass which at unit's distance will have the 

 same energy at the same speed of rotation. The discussion which 

 followed, participated in by a number of those present, brought to 

 light the fact that a good many persons object to the use of the 

 term ' inertia ' at all, some preferring ' moment of mass,' while 

 others were content to retain the term because used iti so many 

 valuable memoirs already in print. 



There was no session of Section A on Friday afternoon nor on 

 Saturday ; and on Monday, no papers being assigned to the section 

 except the two which failed through absence on Friday, and their 

 author still being absent, the section adjourned finally, so far as 

 reading of papers is concerned. 



Section B. 



The address of Vice-Pres. W. A. Anthony before the Physical 

 Section, at its opening on Wednesday, was on the importance to the 

 advancement of physical science of the teaching of physics in the 

 public schools. Professor Anthony took the ground, that, since 

 there is a strong re-action by which the applications of science 



stimulated the development of the science itself by awaking more 

 general interest and by bringing out phenomena that call for ex- 

 planation, the cause of pure science will be advanced by giving in- 

 ventors and all concerned with the application of physics a more 

 thorough training in the principles of that science. 



This training must be given by the public schools, and should 

 begin early, and be prolonged over several years of the course, so 

 that such a principle as the conservation of energy shall not be to 

 the student something which he learned in cramming, during one 

 year or a part of a year, a very difficult subject called physics, but it 

 shall be as familiar and well understood a fact as that water will 

 not flow up hill. When this is the case, the labors of inventors may 

 be expected to be more fruitful, because more intelligently directed, 

 and science itself will be advanced. 



The most importar. pa^. er of the session was presented by Profes- 

 sors Michaelson and Morley, and was on a method of making the wave- 

 length of sodium light the actual and practical standard of length. 

 The methods that have heretofore been advanced for this purpose all 

 depend on the use of the diffraction grating, and do not afford a 

 sufficient degree of accuracy in the comparison ; for, as was re- 

 marked by Professor Rogers, before the wave-length of sodium 

 light can be taken as the standard of length, it must be possible to 

 lay off a distance on a scale which shall represent a given number 

 of wave-lengths, at least as accurately as it is possible to compare 

 two standard scales. Messrs. Michaelson and Morley allow sodium 

 light to fall on a piece of plane parallel glass, where it is divided into 

 two beams at right angles to each other, which are reflected back 

 by two mirrors, and again brought together into one beam, which 

 falls upon the observing telescope. Interference bands are seen, 

 which depend on the difference between the distances which the two 

 beams traverse. One of the two mirrors is provided with a screw 

 motion, by which it can be moved back or forth in the direction of 

 the beam of light which falls upon it. If, now, it is moved, the ob- 

 server at the telescope, by counting the number of interference 

 bands which cross the field of view, can determine the exact num- 

 ber of wave-lengths of sodium light corresponding to that distance. 

 By this arrangement a distance of one decimetre can be directly 

 determined ; and by successively measuring off ten decimetres, 

 and by having the marking diamond rigidly attached to the mirror 

 which is moved, Professors Michaelson and Morley believe that they 

 will be able to lay oft a length of one metre, in terms of a given 

 number of wave-lengths of sodium light, with an accuracy of about 

 one part in a million, which is at least twice as accurately as two 

 metres can be compared ; but, in finding the number of wave- 

 lengths that correspond to a metre, they will of course be limited by 

 the accuracy with which the microscope can be set on the gradua- 

 tions of the standard. 



A second paper of great interest was by these same investigators, 

 and was an account of some experiments by which it was sought to 

 measure the velocity of the luminiferous ether relative to the earth, 

 by the interference between two beams of light, which were reflected 

 back and forth a number of times, one being in the direction in 

 which the ether was supposed to be moving, and the other at right 

 angles to that direction. No effect was found, so that it was con- 

 cluded that the ether must be very nearly at rest with respect to the 

 earth ; but this result leads to serious difficulty in explaining aberra- 

 tion, and should be carefully scrutinized. 



Prof. William A. Rogers, to whose enthusiasm and skill in a 

 very arduous field of research American investigators are so greatly 

 indebted, presented a number of papers to the association, in one 

 of which points of great importance in the exact measurement of 

 lengths were brought out ; in particular, that, in comparing stand- 

 ard scales, a time when the temperature is slowly changing is the 

 worst possible, and that such comparisons should be made either 

 with constant temperature, or at a certain critical time in the day 

 when the temperature is changing quite rapidly, the time depending 

 on the relative masses of the bars and on other circumstances. In 

 the case cited this critical time was about 6 A.M., and measure- 

 ments a half-hour before that time showed errors about equal in 

 amount and opposite in direction to those made a half-hour 

 after it. 



Prof. W. F. Magie, in a study of capillarity, showed reasons for 

 believing that the contact angle of water and glass is not zero. 



