92 



SCIENCE. 



[Vol. I., No. 4. 



can be symmetrically placed. In the case 

 given, the sun's limbs are of the same color 

 and form, and the two positions are symmet- 

 rical with reference to each other. In measur- 

 ing stars, the apparent magnitudes being made 

 approximately equal, their images may be made 

 to pass over each other with the greatest nicety ; 

 and in both these cases the observer's ej'e is 

 steadily directed to a definite point in the tele- 

 scopic field. In practice this seems to give 

 more precise results than when the observer's 

 attention is directed to two points at some dis- 

 tance from each other, and both bisected by 

 the webs of the ordinary micrometer. In the 

 telescope, with such a micrometer, the most 

 exact measurements are not often extended 

 over a minute of arc. And this limit is fixed 

 by the field of view, which decreases as the 

 magnifying power increases. With the heliom- 

 eter, however, the limit of the distance which 

 can be measured is independent of the mag- 

 nifying power and the field of view, but is 

 limited by the amount of motion given to the 

 two halves of the object-glass. In the Yale 

 iieliometer this motion is about two degrees. 

 Another advantage is the absence of either the 

 bright webs or the bright field of the ordinary 

 micrometer ; but this is counterbalanced to 

 some extent by the necessitj^ of making the 

 heliometer object-glass smaller than is usual in 

 equatorials. 



The difficulties and expense of construction 

 of the modern heliometer, the fact that it is a 

 special instrument to be devoted to measuring 

 rather than to viewing, and the less difficulty 

 of manipulation of meridian instruments, and 

 equatorials, led to the comparative neglect of 

 the heliometer by EngUsh-speaking people un- 

 til the erection of the Oxford heliometer of 

 190 mm. aperture. Lord Lindsay's admirable 

 volume (Dun Edit obs. publ., vol. ii.), describ- 

 ing his heliometer of 107 mm. aperture, pre- 

 sented in a very forcible manner the precision 

 attained in measurements with comparatively 

 small instruments. An inspection of the meas- 

 urements executed with the instruments at 

 Breslau (76 mm. aperture), Konigsberg (1.58 

 mm.), Bonn (162 mm.), and Strasburg (76 

 mm.) shows a precision for distances over 1 

 minute not equalled by any other measure- 

 ments made at the same period by instruments 

 of another class. 



The belief that a heliometer of the largest 

 size, and built according to the most recent 

 theories as to material, form, and symmetrical 

 arrangement of parts, would be an important 

 adjunct to the instrumental resources of Amer- 

 ican astronomy, led to the writer's recommend- 



ing to the Yale observatory board the acquisi- 

 tion of such an instrument. 



The contract with the Messrs. Repsold bears 

 the date of June 11, 1880. Tlie heliometer 

 was erected in Repsold' s shops in January, 

 1882, for inspection, and arrived in New York 

 the following May. About the beginning of 

 September it was in place in the west tower 

 of the observatory. 



The figure shows it as erected in Repsold' s 

 shop at Hamburg, and without its tripod foot. 

 The object-glass is mounted in the rectangular 

 metal frame A, which contains the two sliding- 

 pieces holding the object-glass halves, which rest 

 on four cylindrical surfaces each 107 X 13 mm., 

 and having a radius 125 mm. less than the 

 focal length of the object-glass. The large 

 rotating disc B contains three sectors of differ- 

 ent thicknesses of wire gauze, which can be 

 swung over either object-glass half, to diminish 

 the apparent brightness of eitherimage. This 

 whole head can be rotated in position angle by 

 means of the shallow sheet-iron cylinder, which 

 has a rack with its appropriate gearing attached 

 to it. Bj' this device the motion in position 

 angle is as expeditious as in the common form 

 of position micrometer. The position circle is 

 at P. The slow motions and clamps for all the 

 circles are brought within easy reach of the eye- 

 piece E, by a number of ingenious mechanical 

 devices. 



The two small brass oil-lamps, which are 

 carried at the extremities of long arms to avoid 

 their heating effects on the instrument, b}' a 

 careful economy of the light, and a beautiful 

 arrangement of lenses and mirrors, illuminate 

 the object-glass platinum scales, the scale me- 

 tallic thermometer, and both the position and 

 declination circle indices, which are all read by 

 their appropriate microscope micrometers pro- 

 jecting from within the c^'linder C. The tele- 

 scope tube is of steel, the circle graduations 

 are on silver ; the column axes and counter- 

 poises are of iron, and rest upon a massive 

 tripod foot of 0.85 m. radius. The distance 

 from the surface of the granite capstone on 

 which the tripod foot rests, to the intersection 

 of the polar and declination axis, is 2.9 m. 

 The clockwork, with its connecting rod, is 

 shown at D. 



The more important instrumental constants 

 are as follows : aperture, 155 mm. ; focal length, 

 2,495 mm. ; maximum arc to be measured, 2° ; 

 magnifying power of the eye-pieces, 90, 126, 

 159, 245. The scale micrometer has a value 

 of 0".25 for one division of its head ; while the 

 hour circle, declination circle, and position cir- 

 cle micrometer divisions have values of 1% 10" 



