404 C. Bat' us — Methods in Reversed and 



ing the rotation of the micrometer screw, the latter could be 

 used at once. 



In case of homogeneous light and a wide slit, fringes were 

 visible in an ordinary telescope for a play of over 2 cm of the 

 micrometer screw, passing however between extremes of fine- 

 ness. The slit images are not of equal breadth, if first and 

 second order spectra are superposed, but if the longitudinal 

 axes are coincident any position of the narrow image within 

 the broader produces a wide vertical strip of fringes, usually 

 more or less horizontal. They are very easily found. The 

 sodium flame is too feeble for use. The mercury arc is unfor- 

 tunately too nickering, 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 inter- 

 ference 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 sug- 

 gested. The electric arc or a JSTernst filament are equally avail- 

 able as a source of light. Finally by suitably rotating the 

 grating G' on the axis e, by aid of the set of screws d, fringes 

 whose distance apart is over -g- of the width of the telescope field 

 may be obtained, quite sharply. As this distance represents but 

 30 x 10" 6 cm., there is no difficulty of realizing 10~° 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 for orientation. Accordingly 

 the Fraunhofer micrometer, reading to 10" 4 cm., was provided 

 at its screw head with a light wooden wheel, w, fig. 4, about 

 10 cm in diameter and 3 millimeters 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 millimeters 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. 



The fringes should be widened as far as convenient, by 

 rotating the grating on the axle e, fig. 2, by aid of the set 

 screws, d. In this case they climb up or down the transverse 

 strip, as s in fig. 4 is slowly rotated. Fringes moving horizon- 

 tally are not serviceable, because they are too near together. 



