74 



THE INTERFEROMETRY OF 



faces confronted the half -silvered surface on ordinary plate glass. Conse- 

 quently, the fine fringes were observed by transmitted light behind, and the 

 medium and coarse fringes by reflected light in front. The micrometer was 

 a good instrument for general purposes, but hardly equal to the present work, 

 where the slightest rocking of the slide introduces annoyances. 



To count the number of fringes between D and b, since the fringes were not 

 generally seen in the principal focal plane of the telescope, it was considered 

 sufficient to rotate the cross-hair into an oblique position, until its ends ter- 

 minated in the D and b lines, respectively, and then to count the number of 

 fringes on running the eye down the wire from end to end. When there are 

 many fringes, 25 to 50, the eye is apt to tire before reaching its destination, 

 so that several counts must be made and the mean taken. 



50 







-01 



The results are given as a whole in figure 54, where the distance between 

 plates, measured in centimeters on the micrometer, beginning at an approxi- 

 mate zero, is laid off horizontally and the number of fringes vertically, in case 

 of each of the three series. The computed line e = Cn'/2 is drawn in full and 

 the observations laid off with regard to it. The zeros do not quite correspond, 

 as very small distances here are significant. With the fine fringes I did not 

 spend much time, as they are virtually colors of thin plates seen by diffrac- 

 tion. The chief difficulty with these small distances is that the plates touch 

 and a complete readjustment is necessary. After touching, the micrometer 

 acts like a forcing screw and its reading is too low. This is the meaning of 

 the data in the curves a and a', the latter with its horizontal scale magnified 

 ten times. The object of this series is chiefly to locate the position of the 

 line in relation to the other lines. 



