94 



SCIENCE. 



[Vol. I., No. 4. 



and 10" respectively. The aperture of the 

 finder is 62 mm. The whole instrument is an 

 exquisite piece of mechanical workmanship, 

 and for both design and execution the makers 

 are entitled to the highest praise. 



The construction of the object-glass, first of- 

 fered to the Clarks, but declined by them be- 

 cause of the subsequent cutting in two, was 

 accomplished by Merz of Munich. Its per- 

 formance, using the Steinheil achromatic eye- 

 pieces, is an agreeable surprise. With either 

 half the images are as sharp as with a good 

 four-inch telescope. With the images super- 

 posed, there is a loss of the best definition ; and 

 this arises from the practical impossibility of 

 adjusting the two halves of such an object- 

 glass so that the images will be absolutely 

 superposed. In actual observing, the greatest 

 difficultj' in the waj' of exact measurement is 

 found in a want of similarity in the atmos- 

 pheric conditions affecting two celestial ob- 

 jects which are supposabl^^ near enough to be 

 influenced alike. Thus the two opposite limbs 

 of the sun, except in the very best observing 

 weather, do not maintain a steadj' contact 

 together when heliometricallj' observed, but vi- 

 brate, alternately lapping over and receding 

 from each other. In the observations of the 

 last transit of Venus, this peculiarity presented 

 the curious effect of a rapid breaking and form- 

 ing of a ligament analogous to the ' black drop ' 

 described by the older observers when the 

 limbs of Venus and the sun were in contact. 



The model on which the whole instrument 

 is constructed is a very great improvement on 

 an}' previous heliometer, so far as lessening 

 the observer's fatigue is concerned. Every 

 motion is controlled, and every scale and cir- 

 cle is read, by the observer without leaving his 

 seat. 



With the cylindical bearings of the object- 

 glass cells, the image distortion for measures 

 up to 2° is rendered extremely small ; bj' the 

 rapid rotation in position angle, and equal 

 rapidity in distance settings, the observer is 

 no longer fatigued hy manipulation ; and it can 

 be said that in this instrument the heliometer 

 shows itself to be a measuring-machine of the 

 highest precision. Leonard Waldo. 



NOTE ON THE OBSERVATIONS OF THE 

 TRANSIT OF VENUS, ISSS, AT THE 

 LICK OBSERVATORY. 



By invitation of Capt. E. S. Floyd, presi- 

 dent of the trustees of the James Lick trust, 

 I went to Mount Hamilton to direct the obser- 

 vations of the transit of Venus at the Lick 



Observatory. The chief instrument of the 

 equipment which the trustees had provided in 

 time for observing the transit was the horizon- 

 tal photoheliograph, which is essentially simi- 

 lar to those emploj'ed by the American com- 

 mission on the occasion of the transit of 1874, 

 as well as that of 1882, and which are described 

 b}' Professor Newcomb in the first part of the 

 American observations of the transit of Venus 

 of 1874. The Lick photoheliograph, like all 

 the others, has an objective five inches in diam- 

 eter ; and its focal length is almost exactly 

 forty feet. The heliostat mirror, an unsilvered 

 disk of glass, is seven inches in diameter, and 

 was mounted on a pier adjacent to that which 

 supported the objective. A third pier, coming 

 up in the interior of the photographic house, 

 supported the plate-holder ; and all three piers 

 were laid up of brick, their foundation being 

 in the rock of the mountain summit. . 



I arrived at the observatory in the evening 

 of Nov. 21. The photoheliograph had, in the 

 main, been mounted and got in readiness before 

 that time by Mr. Fraser, the superintendent 

 of construction of the observatorj'. It re- 

 mained to complete the unfinished portions of 

 the instrument, to mount and fully adjust the 

 same, to modify some details which had been 

 unsuitably constructed, and to make sure of 

 the convenient and effective working of every 

 part. Especial attention was given to the 

 accurate determination of the position of the 

 focal plane of the objective ; and the method 

 adopted — being nothing short of a critical 

 examination, by manj' persons independently, 

 of several sets of trial-plates exposed at vary- 

 ing distances from the objective — finally indi- 

 cated the true setting of the plate-holder with 

 much more than the required precision. Great 

 care was taken to insure the perfect definition 

 and figure of all the pictures, and to prevent 

 the mishap of fogged plates from scattering 

 and extraneous light. Much time was consumed 

 in this wa}' in the preparatory' work, but we 

 had more than sufficient compensation in the 

 superior character of the photographs of Venus 

 in transit. All these were taken b}' the wet 

 process, and the photographic operations were 

 in charge of Mr. Lovell of Amherst. 



During the important days of the transit- 

 period, the meteorological conditions on Mount 

 Hamilton were especially favorable. At mid- 

 night, Nov. 30, the sky cleared, after three 

 and a half days of continuously cloudy weath- 

 er. From that time until the afternoon of 

 Dec. 7 we saw no cloud, daj' nor night, which 

 could in the kast interfere with anj' observa- 

 tion we had to make. Thin cirrus was float- 



