ASTRONOMY — HAI,E. 8l 



urement of rectangular or polar coordinates on lo by 12 plates ; a small 

 Gaertner machine for stellar spectra; the heliomicrometer ; a Hartmann 

 microphotometer by Toepfer; a special instrument for measuring the 

 area and intensity of the flocculi ; and two calculating machines. On 

 Mount Wilson there is also a Zeiss stereocomparator, with monocular 

 micrometer attachment, and a small measuring machine. 



Reference has already been made (p. 69) to various methods which 

 have been employed in the measurement and discussion of spectro- 

 heliograph plates. In the study of the solar rotation, as indicated by 

 the motion in longitude of the flocculi, the important requirement is a 

 means of measuring heliographic positions. Photographs taken at inter- 

 vals ranging from ten hours to two or three days are compared with the 

 stereocomparator, and those points in the flocculi which show the smallest 

 changes in form are marked for measurement on the successive plates. 

 The method for obtaining heliographic positions at present in use at 

 Greenwich and elsewhere involves the measurement of the polar coordi- 

 nates of the points in question, followed by a computation which neces- 

 sarily occupies considerable time. In routine work requiring the meas- 

 urement of from 50 to 75 points on each one of several thousand plates, 

 the employment of some method which would eliminate all computa- 

 tions is evidently most desirable. The globe-measuring machine, recently 

 named the "heliomicrometer," was accordingly devised for this purpose. 

 The form of this machine, briefly described and figured in the last annual 

 report, required the use of two 4-inch telescopes, each of 60 inches focal 

 length. One of these telescopes was directed toward the photograph of 

 the sun, placed at a distance of 60 feet. The other was directed toward 

 a globe, at approximately the same distance. By means of suitable re- 

 flectors the images of the globe and plate given by the two telescopes 

 were united in a single eyepiece. The flocculi were then seen upon the 

 surface of the globe, which was ruled with meridians and parallels one 

 degree apart. By setting the axis of the globe parallel to the axis of the 

 sun for the date of the photograph, the heliographic latitude and longi- 

 tude of any point on the photograph could be read off directly, by estimat- 

 ing its position with reference to the neighboring meridians and parallels. 



A measuring machine of this form was constructed in our Pasadena 

 instrument shop. In experimenting with it, it occurred to me that it 

 would be far better if a cross-hair were set on the point to be measured, 

 before the image of the globe had been thrown into the field of view. In 

 this way settings could be made on the most delicate details of the photo- 

 graph, which would be lost to view when observed in projection on the 

 illuminated surface of the globe. The method still involved, however, 

 the undesirable element of estimating the position of the cross-hairs with 



6 — YB 



