April 20, 1883.] 



SCIENCE. 



301 



For observing contacts I used an ej-e-piece 

 magnifying 216 diameters, attached to a Her- 

 scliel solar prism, and a sliding-shade glass 

 with a density varj'iug uniform^ from end to 

 end. The limb of the sun was remarliably 

 stead}-. The assistant astronomer, Lieut. J. 

 A. Norris, U.S.N. , was to take the time of my 

 signals from a mean-time chronometer, while 

 with an obserxdng-kej" I was to make a record 

 on the chronograph as a check. 



About forty seconds before the computed 

 time of first contact, a narrow stratus cloud 

 passed upon the south-eastern edge of the sun, 

 and shut out all the light. The cloud remained 

 about three minutes ; and, when it passed off, 

 the notch in the sun's limb was plainlj- marked. 

 Two photographs were taken to test the appa- 

 ratus and the plates ; and then the time before 

 second contact was devoted to an examination 

 of the limbs of Venus and the sun. Both were 

 perfectly steadj'. In observations of the sun 

 for the last twentj- years I never saw it better. 

 At about thirteen minutes after first contact, 

 the outline of the entire disk of Venus could be 

 seen, and seemed perfectly- circular. About 

 two minutes later, a faint, thin rim of yellowish 

 light appeared around the limb yet outside the 

 sun. This rim was at first broadest near the 

 sun's limb, but soon the width of the light 

 became uniform throughout. The light was 

 wholly exterior to the limb of Venus ; i.e., the 

 black limb of Venus on the sun, and the dark 

 limb outside, formed a perfectly circular disk 

 with the rim of light, or halo, outside the por- 

 tion off the sun. As the time of second con- 

 tact approached, Lieut. Norris again took up 

 his station at the chronometer. As the limbs 

 ucared geometrical contact, the cusps of sun- 

 light began to close around Venus more rapidlj' ; 

 and the perfect. definition of the limbs, and the 

 stead}-, deliberate, but uniform!}- increasing- 

 motion of the cusps, convinced me instantl}- 

 that the phenomena attending the contact 

 would be far more simple than I had e^-er 

 imagined. I had only to look steadih-, to see 

 the cusps steadily but rapidly extend them- 

 selves into the thinnest visible thread of light 

 around the following limb of Venus, and 

 remain there without a tremor or pulsation. 

 At the moment the cusps joined I gave the 

 signal, and also made tlie record on the chrono- 

 graph. Still keeping my eye at the telescope, 

 I saw nothing to note save the gradually in- 

 creasing line of light between the limbs of the 

 two bodies. The disk of Venus on the sun 

 was black. All the apparatus connected with 

 the photographic work was again examined ; 

 and, at about ten minutes after second contact, 



each member of the part}- was at his station. 

 Lieut. Norris, who had charge of the chrono- 

 graph and the heliostat, was stationed at the 

 latter instrument to see that at certain inter- 

 vals the motion of the heliostat was corrected, 

 and the sun's image thrown in the proper 

 direction. In the photograph-house, the as- 

 sistant photographer, Mr. G. Maxwell, took 

 each plate from the bos, placed it in tlie plate- 

 holder, called its number, and, after exposure, 

 returned the plate to the proper box. My own 

 share of the work was to record the number of 

 the plate, move the esposing-slide, record the 

 time of exposure of each plate from a chro- 

 nometer as a check on the chronograph record 

 and as a means of identification, and commu- 

 nicate with Lieut. Norris b}' a s^ystem of 

 signals on the measuring-rod. The chief pho- 

 tographer, Mr. G. Prince, developed the last 

 plate exposed until nearh' all the clouds had 

 disappeared, carefull}- watched all the photo- 

 graphic manipulations, advised in regard to 

 the length of exposure, etc., and prepared and 

 developed, with occasional aid from Mr. Max- 

 well, all the wet plates used during the day. 

 After the clouds disappeared, measures of the 

 diameter of Venus were made with a double- 

 image micrometer attached to the 5-inch tele- 

 scope; and then tlie photographic work was 

 resumed more leisurely. It was intended to 

 use dry plates for all the work ; but diflSculty 

 in drying tire first 150 which were coated, led 

 me to the determination to coat anew onl}- 150 

 l^lates, and leave the others to be used as wet 

 plates if the dr}- plates should unexpectedly 

 fail at the last hour. After eleven o'clock a.m. , 

 the clouds disappeared ; and, finding we had 

 plentj- of time on our hands, we exposed six 

 wet plates after each group of twelve dry 

 plates. 



At about ten minutes before third contact 

 w-e -had exposed 150 dry plates and 30 wet 

 ones. The majoritj- of the dr}- plates were 

 exposed with a slit 1.5 inches wide, while with 

 the wet plates the width was three-eighths of 

 an inch. On going to the telescope to observe 

 the last contacts, I found the limbs of Venus 

 and the sun as steadj- as in the morning ; and, 

 thougli there was now some haze over the sun, 

 it did no harm. The third contact was ob- 

 served with great accuracy, nothing occurring 

 to obstruct or complicate the verj- simple and 

 definite phenomena which was in the reverse 

 order of that seen at second contact. The 

 rim of light appeared around Venus as soon 

 as the limb was visible beyond the sun, and 

 was seen for nearlj- ten minutes. The com- 

 plete outline of Venus was visible for two 



