THE AID OF THE ACHROMATIC FRINGES. 



81 



TABLE 4. Gravitational attraction. M = 950 grams, m = o.6l gram. R = 4.2cm. Period 

 (damped) about 18 min. Length of needle, 25.2 cm.; weight (total), 1.9 grams. 



* Out of field of telescope. 



As the micrometer reads to io~ 4 cm., 1/1300 part of the attraction between 

 M and m = o.6i gram could therefore be detected; i.e., the attraction of 

 950/1300 = 0.73 gram, or per interference fringe well within one-third of this, 

 for the given quartz fiber, which was not specially selected, and distance 

 R. This is equivalent to the attraction of two tenth-gram masses per centi- 

 meter of distance per fringe. 



Apart from the measurement of the torsion coefficient of the fiber, there 

 is, however, a real difficulty involved, and that is the occurrence of marked 

 drift in the needle. It is only incidentally that the fringes are found at rest. 

 The chief contributory cause of this is no doubt the occurrence of motion of 

 air around the needle provoked by small differences of temperature, result- 

 ing (for instance) from illumination. If the possible accuracy of deflection 

 measurement is to be of any value, therefore, the apparatus must be kept in the 

 dark, except during observation . Fortunately, the achromatic fringes require 

 little light. Even then a closed case which can be exhausted of air is essen- 

 tial, for such radiometer forces as may enter would in any event be differential, 

 seeing that the mirrors are symmetric and the illumination is not subject 

 to alternations like those in the table. It is probable that in case of the above 

 regular method a thicker quartz fiber and a greater distance R would con- 

 duce to the best results, since A N is the least difficult quantity to determine. 



Measurements could not be attempted in time for the present report, but 

 the question may be asked whether it is not possible in the present case to 

 6 



