REVERSED AND NON-REVERSED SPECTRA. 



11 



-arc is unfortunately too flickering, so that the fringes jump about and are 

 useless for measurement. Excellently sharp quiet fringes are obtained with 

 sunlight (white), in which the cross-hatched interference pattern is nearly 

 linear at the line of symmetry of the reversed spectra. The fringes climb 

 very decisively up and down this line with the motion of the micrometer, 

 reduced as suggested. The electric arc and a Nernst filament are equally 

 available as a source of light. Finally, by suitably rotating the grating G', 

 figure 2, on the axis e, by aid of the set of screws d, fringes whose distance 

 apart is over one-third of the width of the telescope field may be obtained 

 quite sharply. As this distance represents but 30 X icr 45 cm., there is no diffi- 

 culty of realizing lo" 6 cm. in case of these long fringes. 



3. Measurements. First= and second-order spectra. The steadiness of the 

 fringes, even in an agitated location, induced me to make a few measurements 

 ior orientation. Accordingly, the Fraunhofer micrometer, reading to icr 4 cm., 

 was provided at its screw-head with a light wooden wheel w, figure 4, about 



10 cm. in diameter and 3 mm. thick. A groove was cut in the circumference 

 of the wheel, so that a silk thread t could be wrapped around it. The other 

 end of the thread was wound around a brass screw s, about 6 mm. in diameter, 

 turning in a nut, preferably of fiber, which was fastened to the edge of the 

 table by a small brass clamp. In this way it was possible to control the motion 

 of individual fringes crossing a fiducial line in the field of the telescope. This 

 simple device worked surprisingly well, a smoothly running micrometer being 

 presupposed. In fact, it was possible to set a fringe to a few millionths of a 

 centimeter. Later the micrometer-head was grooved and a finer turning- 

 screw suitably attached to the base B of the apparatus. (Cf. 70, below.) 



The fringes should be widened as far as convenient, by rotating the grating 

 on the axle e, figure 2, by aid of the set-screws d. In this case they climb up 

 or down the transverse strip as 5 in figure 4 is slowly rotated. Fringes moving 

 horizontally are not serviceable, because they are too near together. It is 

 not difficult to obtain the single vertical line, black or bright, on suitable 

 rotation about e. On either side of this transitional adjustment the fringes 

 move vertically (climb or fall) in opposite directions for the same micrometer 



