120 RECENT PROGRESS IN OPTICS. 



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, etc., are 

 thus compared, and finally in this way the value of the meter is reached. 



The feasibility of this ingenious method having been made apparent, 

 Michelson was honored with an invitation from the International Bureau 

 of Weights and Measures to carry out the measurement at the obser- 

 vatory near Paris, with the collaboration of the director, M. Benoit. 

 After many months of labor, results of extraordinary accuracy were 

 attained. For the red line of cadmium at an air temperature of 15° C. 

 and pressure of 760 mm., two wholly independent determinations were 

 made. From the first a meter was found equal to 1,553,162.7 wave 

 lengths; from the second, 1,553,164.3 wave lengths, giving a mean of 

 1,553,163.5 the deviation of each result from the mean being very nearly 

 one part in two millions ("Travaux et Memoires du Bureau Interna- 

 tionale des Poids et Mesures," Tome XI, p. 84, 1894). A determination 

 by Benoit from the first series gave 1,553,163.6, which differs but one- 

 tenth of a wave length from the mean of Michelson's measurements. 



The direct comparison of the lengths of two meter bars, though not 

 easy, is a simple operation in comparison with the indirect method just 

 described, but does not surpass it in accuracy. Everyone knows that 

 the meter is not an exact submultiple of the earth's circumference, and 

 that the determination of its exact value from the seconds pendulum 

 is full of difficulty. It may perhaps be said that the optical method is 

 no more absolute than the x^endulum method, for no human measure- 

 ments can be free from error; that there is no possibility of the 

 destruction of the original meter and all certified copies of it; and 

 that there is no proof or probability that molecular changes are grad- 

 ually producing modifications in standards of length. Even if we 

 should grant that for all practical purposes the labor of determining 

 the meter in terms of an unchanging optical standard has been unneces- 

 sary, the achievement is a signal scientific triumph that ranks with the 

 brilliant work of Arago, Fresnel, and Begnault. In preparation for it 

 much new truth has been elicited, and light waves have been shown to 

 carry possibilities of application that Fresnel never suspected. 



The physicist is nearly powerless without the aid of those who possess 

 the highest order of mechanical skill. The interferential comparer 

 could never have been utilized for such work as Michelson has done with 

 it, had not Brashear made its optical parts with such an approach to 

 perfection that no error so great as one-twentieth of a wave length 

 could be found upon the reflecting surfaces ("Travaux et Memoires 

 du Bureau Internationale des Poids et Mesures," Tome XI, p. 5, 1895). 

 In the conception, mechanical design, and execution the entire work 

 has been distinctly American. 



The interferential refractometer has been used Avith much skill by 

 Hallwachs (Wiedemann's Annalen, Band 47, p. 380, and Band 53, p. 1) 



