THE AID OF THE ACHROMATIC FRINGES. 



39 



without much distinction between fringes. On being made transverse to 

 the white image of the fine slit, they cross-hatched it from top to bottom. 

 Nevertheless, their rapidity of motion is such that they serve quite well for 

 measurement, the datum being more accurate than the measurement of 

 Aa. The comparison was carried out in the same manner as before, the pres- 

 ence of the achromatics being successively destroyed by rotation (Aa) and 

 restored by the normal displacement (A7V) of the mirror at n. In this way the 

 following data among many others were obtained. It is necessary to dis- 

 place the mirrors very carefully; for if the fringes are lost they are extremely 

 difficult to find without beginning with the spectrum fringes all over again. 



The range of Aa is much increased by removing the objective lens of the col- 

 limator, and this is done after the observation marked x in the table. The 

 fringes are perhaps even more distinct when present in the absence of the 

 lens. The constants of the apparatus were: 



6 = 21 cm.; 18 = 70.7 ; ^ = 64 cm. 



From this the rate AAf/Aa= 1.05 was found graphically. In the other series 

 the rates were above 0.9. Approximate estimates of the same value were ob- 

 tained with the spectrum ellipses and the glass column. This result again 

 differs from the computed value, AN = o. The reason may lie in the fact that 

 the plane of symmetry of the prism P (fig. 14) did not pass through the axis 

 of rotation, or was not originally midway between the mirrors m and n. To 

 test this inference (which will again be treated in the next section) the fol- 

 lowing experiments were made: 



The prism P was as carefully as possible centered by the eye, so that its 

 plane of symmetry passed through the axis of rotation. In this case the 

 relative measurements 



icfi AN = 0.0 



10'Aa =O.O 



1-5 



2.2 



4-5 

 4.8 



6.6 

 7-4 



8.1 cm. 

 9-3 



showed a mean coefficient of AN/Aa = o.8j. Finally the prism was moved 

 to the right, i. e., with its plane of symmetry on the other side of the axis. 



The results were 



io'AN=o.o 3.0 4.4 7.0 cm. 

 io 3 Aa =0.0 3.0 4.4 6.7 



giving a mean rate AA/"/Aa = 1.05. Thus the shifting of the prism right and 



left has made but little difference and can not account for the discrepancy. 



It is probable that the coefficients found are largely due to the half-silver 



glass mirror n (fig. 15), which rotates with the rail mn. To test this a com- 



