352 SMITHSONIAN MISCELIvANUOUS COLIvECTIONS VOX,. 52 



these points, referred to the center of the Sun. As the flocculi are 

 seen in projection on the surface of a sphere, it is evident that a 

 considerable amount of calculation would be required to deduce the 

 latitudes and longitudes if the ordinary methods of measurement, 

 giving their distance along a radius from the center of the disk, and 

 the angle between this radius and the north pole of the Sun, were 

 employed. To obviate this computing, the heliomicrometer was de- 

 vised for the measurement of these photographs, and constructed in 

 the instrument shop of the Solar Observatory. This instrument 

 consists essentially of two 4-inch telescopes, one of them pointed 

 at the solar photograph, the other at a silvered bronze globe, placed 

 near it. By a suitable device the images given by the two telescopes 

 are brought together in a single eye-piece, so that the observer sees 

 the photographs projected upon the surface of the globe. If, then, 

 the globe is ruled with meridians and parallels one degree apart, 

 and the axis of the globe is inclined at such an angle as to correspond 

 with that of the Sun on the date of the photograph, it is evident 

 that the latitude and longitude of any point on the photograph can 

 be read ofif to a tenth of a degree, with reference to the nearest 

 meridian and parallel. In practice, many refinements are introduced 

 to increase the precision of measurement. For convenience, the two 

 telescopes are mounted immediately above the globe and photo- 

 graphic plate and pointed at two plane mirrors 30 feet away, in 

 which the globe and plate are seen. It has been found that the 

 rapidity and precision of measurement with this instrument are as 

 great as with the ordinary method, while all of the extensive compu- 

 tations are eliminated. 



During the summer season of 1907 the Sun was photographed 

 with the Snow telescope on 113 consecutive days. Such an un- 

 broken series of negatives provides the best of material for the study 

 of changing solar phenomena, since the successive phases can be 

 observed without the interruptions encountered under less favorable 

 atmospheric conditions. In the determination of the solar rotation, 

 for example, a cloudy period of two or three days may prevent the 

 measurement of a large proportion of the calcium flocculi, because 

 their changes of form are so rapid. 2,585 positions of calcium 

 flocculi have been measured on 76 plates, by Miss Ware, with the 

 heliomicrometer, and the combined results furnish the following 

 values for the rotation periods of the calcium flocculi at different 

 latitudes. 



