MOUNT WILSON OBSERVATORY. 205 



observed, the result would be difficult to reconcile with the theory of relativity, 

 but would point rather to an ether completely or partially dragged along by the 

 earth in its rotation. Experiments were made by Michelson on Mount Wilson 

 with two circuits, 6,500 and 5,200 feet in length, and although fringes were 

 observed, their quality was such as to preclude the possibility of accurate 

 measurement. In these tests the path of light was close to the level of the 

 ground, and it is possible that a site might be found for which the conditions 

 of seeing would be greatly improved. If this could not be done, it is probable 

 that Michelson's suggestion of a pipe line exhausted of air would offer a definite 

 solution of the problem (p. 244). 



The use of an iris diaphragm not far from the focus of a telescope has been 

 suggested by Michelson as a means of securing objective measurements of 

 conditions of seeing. The aperture of the diaphragm is diminished, thus 

 diminishing the effective aperture of the telescope until diffraction rings 

 appear. Tests made by van Maanen with such a diaphragm on the 60-inch 

 telescope show that the simple relationship 



£'=1-0.15 d 



where >S' is the seeing on a scale of 10 and d the effective aperture of the mirror 

 in inches, lepresents the results of observation in a satisfactory way (p. 245). 



The solar activity has continued to diminish, and many days now occur 

 on which no sun-spots are visible (p. 210). Of the 33 spot groups observed 

 during the first four months of 1922, 4 were of irregular polarity. Although 

 this high percentage (10) of irregular spots might be due to the small num- 

 ber observed, a close watch of all spots has been kept as the minimum 

 approaches. As a result a small spot was observed by Ellerman on June 24 

 which is probably the forerunner of the new spot cycle. Its latitude was 

 31° north and its polarity was opposite to that of regular preceding spots 

 in the northern hemisphere. 



Messrs. Hale, Ellerman, Nicholson, and Miss Mayberry have detected 

 invisible sun-spots by searching for evidences of the Zeeman effect in prom- 

 ising regions, such as areas of flocculi following a large spot. A special 

 polarizing apparatus permits very small magnetic fields to be found by the 

 alternate widening to red and violet of the iron triplet X6173. These obser- 

 vations confirm the view that a spot represents a vortex, which becomes 

 visible only when the cooling due to expansion is sufficiently great to produce 

 a perceptible decrease in the brightness of the photosphere (p. 212). 



The continued study of the Zeeman effect in sun-spot spectra by Hale, 

 Nicholson, and Ellerman has yielded further results bearing on the variation 

 of the field-strength at different levels. Nicholson has proved that the ap- 

 parent displacements of the p-component of spot triplets on photographs 

 taken with a compound quarter- wave plate are subjective phenomena, due 

 to the photographic influence of closely adjoining lines. The results are also 

 important in their bearing on other classes of spectroscopic work (p. 212). 



The results of the cooperative efforts of a number of observers of standards 

 of wave-length are seen in the adoption at the Rome meeting of the Inter- 

 national Astronomical Union of a list of 305 tertiary standard lines in the arc 

 spectrum of iron to be used as reference lines on the international system of 

 wave-lengths. The values adopted depend upon an average of five or six 



