82 REPORTS ON INVESTIGATIONS AND PROJECTS. 



respect to the rulings on the globe. In order to render the method of 

 the highest precision, it was evident that the substitution of divided 

 circles for the ruled surface of the globe should prove most advantageous. 

 The adoption of this conclusion led to the reconstruction of the machine. 

 In its present form, the two telescopes of the heliomicrometer are 

 mounted immediately above the globe and photographic plate. Two 

 plane mirrors, placed at a distance of 30 feet, receive light from the plate 

 and from the illuminated surface of the globe, and reflect it to the two 

 telescopes. All motions of adjustment, for centering the plate on the 

 globe, setting the cross-hairs on the object to be measured, rotating the 

 globe, etc., can be effected by the observer from his seat at the eye-end 

 of the telescope. In the operation of measurement, the cross-hairs are first 

 set on the flocculus and the globe is then illuminated. The surface of 

 this globe is not ruled with meridians and parallels, but simply bears a 

 small circular dot at the intersection of the equator and the central merid- 

 ian. By rotating the globe in longitude and in latitude, this dot is made 

 to coincide with the intersection of the cross-hairs. It is then only neces- 

 sary to read off, on circles provided for the purpose, the latitude of the 

 flocculus and its dift'erence in longitude from the center of the sun. It 

 has been found by comparative tests that the operation of measurement 

 with the heliomicrometer requires no more time than with an ordinary 

 measuring machine giving polar coordinates. The resulting precision is 

 equally great, and all computations are avoided. The heliomicrometer 

 is easily transformed into a type of stereocomparator, for comparing two 

 plates, by mounting a second photograph of the sun in front of the globe, 

 and illuminating it from behind. In this way the flocculi on successive 

 plates are selected for measurement and marked by means of an electric 

 pen, controlled from the eye-end of the telescope. The instrument also 

 has other useful applications in connection with the reduction of solar 

 photographs. 



Three computers are now employed under the direction of Mr. Adams. 

 The routine work comprises : 



(i) Measurements of heliographic positions of flocculi with the helio- 

 micrometer, for the determination of the solar rotation and other pur- 

 poses. 



(2) Measurements of the area and brightness of the flocculi in regions 

 10 degrees square on the solar surface, for the determination of the solar 

 activity and its variation with the sun-spot period. 



(3) Studies of the lines affected in photographs of the spectra of sun- 

 spots. 



(4) Studies of the changes in relative intensity of lines in spectra 

 obtained at high and low temperatures. 



