13.6 ANNUAL OF SCIENTIFIC DISCOVERY. 



retarding plate converts the elliptically polarized light, which has to be 

 examined, into plane polarized ; and this plane polarized light is extin- 

 guished by the Nicol's prism. There are two distinct positions of the 

 retarding plate and the Nicol's prism in which this takes place. In 

 each of these principal positions the retarding plate and the Nicol's 

 prism maybe reversed (/. e., turned through 180), and the means of 

 the readings in these four subsidiary positions may be taken for greater 

 accuracy. The readings of the fixed and movable verniers in each of 

 the two principal positions are four quantities given by observation , 

 which determine four unknown quantities ; namely, 1. The index error 

 of the fixed verniers, or which comes to the same, the azimuth of the 

 major axis of the ellipse described by the particles of rether, measured 

 from a plane fixed in the graduated circle. 2. The ratio of the axis of 

 the ellipse. 3. The index error of the movable verniers. 4. The re- 

 tardation due to the retarding plate. The unknown are determined 

 from the known quantities, by certain simple formulae given by the 

 author. The author stated that he had made a good many observa- 

 tions with this instrument for the sake of testing its performance, and 

 that he had found it very satisfactory. Inasmuch as light is not homo- 

 geneous, the illumination never vanishes, but only passes through a 

 minimum, and in passing through the minimum the tint changes rap- 

 idly. This change of tint is at first somewhat perplexing ; but, after 

 a little practice, the observer is able to know it mainly by intensity, 

 taking notice of the tint as an additional check against errors of obser- 

 vation. The accuracy of the observations is a little increased by the 

 use of rather -pale-colored glasses. To give an idea of the degree of 

 accuracy of which the instrument is susceptible, suppose the ratio of 

 the axes of the ellipse described to be about three to one. In this case 

 the author found that the mean error of single observations amounted 

 to about a quarter or the fifth part of a degree in the determination of 

 the azimuth, three or four thousandths in the determination of the 

 ratio of the minor to the major axis, and about the thousandth part of 

 an undulation in the determination of the retardation. On account 

 of the accuracy with which the retardation is determined, and the 

 largeness of the chromatic variations to which it is subject, the instru- 

 ment may be considered as determining not only the elements of the 

 ellipse described, but also the refrangibility of the light employed, or 

 its length of wave, which corresponds to the refrangibility. The author 

 stated, that the error of the thousandth part of an undulation, to which 

 the determination of the retardation was subject, corresponded to an 

 error of only the twentieth or thirtieth part of the interval between the 

 fixed lines D and E of Fraunhofer. 



ON THE APPARENT MOTION OF CERTAIN COLORED FIGURES. 



PROF. LOOMIS, at the American Association, Albany, presented a 

 communication, relating to the apparent motions of certain colored fig- 

 ures, a phenomenon generally known under the name of the " dancing 

 mice." The phenomenon to be explained is this: When a green 

 figure or stripe is worked on a red ground, and the card gently agitated, 



