126 



THE INTERFEROMETRY OF 



the second set the sun was focussed with a weak lens (0.5 meter in focus) at 

 the point formerly occupied by the electric arc. The spectrum (particularly 

 in the second case) was brilliant and the lines clear. The focus of sunlight is to 

 be placed just outside the focus of the collimator lens, in order that a nearly 

 linear pencil may be available to penetrate the long refraction tube twice. 

 The distance of the collimator lens to the grating was about 2 meters. The 

 spectrum is then a bright band in the telescope, the width being limited by the 

 height of the ruled part of the grating. The strip of white light on the grating 

 should not be more than a few millimeters wide. It must therefore be nar- 

 rowed by an opaque screen (wide slit of the given width) in the path of the 

 beam (see fig. 92 below). 



TABLE 9. Dispersion of air. Tube /= 138.0 cm. Bar. 77.25 cm. at 19.5. P=75-O cm 



The equation for B in this case, if the symbol 8 refers to differences for two 



given values of X, is 



_ 8&N/e 76 r 



*i P 273 



if the value of B is to hold for normal conditions. 



The data are shown in table 9, series i and 2 being obtained without con- 

 densing lens. These are inferior, as regards definition of lines, to the subse- 

 quent set, in which condensed sunlight was used. In all cases there is some- 

 times an irregularity (marked by ? in table 9) in which the observation is 

 obviously discordant, but the reason could not be found. Possibly values of 

 T and p taken were not the actual values. The data for BX io 14 given in the 

 table are mean values. Some of these are low. Later values, where the F 

 line is included, come out larger, the range being from 1.3 to 1.8 or 1.5, on 

 the average. It is desirable to use the whole of the available range of the 

 spectrum (sufficiently luminous from C to F) to obtain an acceptable value 

 of the coefficient B and additionally to improve the method by using two 



