TRANSIT OF VENUS.

Globe, Volume II, Issue 187, 14 January 1875, Page 3

TRANSIT OF VENUS.

At a banquet to the members of the American and French Transit of Venus Expedition, given in Dunedin a few days ago, Professor Peters gave some account of the observations made by his party. We make the following extract from his speech:— In foimer times—in the last century—when the previous observations of the transit were taken, there were no other means available but, to follow the rules of Halley or De Lisle—to measure the time when Venus would be seen in contact with the sun—when the limbs touched either externally or internally. Since that time other sciences had come into existence, and as every science helped the other, these sciences helped astronomy. For instance, they now had photography, and when the time was approaching that the transit of Venus should again take place, one of their first ideas was to make use of photography to determine this small displacement. He would explain how the measurements of photography stood in regard to the former measurements. It was necessary to mark the exact time when Venus’s centre was a certain distance from the sun’s centre. Let them suppose the sun’s centre and Venus’s were in a vertical line—then the displacement would be such that the distance of the centre would be either shortened or lengthened according as the observer stood in the northern or southern hemisphere. If Venus and the sun were on the same elevation above the horizon, the parallax would feel the effect of not being shortened 1o any considerable extent —in fact, the distance would not be affected at all, but it would have an effect upon the angle. The greatest effect would be upon the angle ; and they had the means of computing the angle which the line from the sun’s centre to Venus’s made with a certain given line upon the sphere. This displacement was formerly measured alone ; so in the contact they observed how much longer one party observed the contact after anoth r. If photography had been applied, the dis-

tance would have been measured on a photographic plate - t lie distance of the angle of position. They -• tvv, therefore, that by photography they h* , .if, the means of mul tiplying immensely one ohsei vaMoti the contact, and one observation by photography equalled, under the same circumstances, one contact. Therefore, the American expedition directed their efforts to make the photographic observation as perfect as possible and considered the contact observation only, as a confirmation, or rather as an observa tion necessary for examining certain physical phenomena connected with the contact. The principal photographic apparatus was a lense of sin aperture, and 40fc local distance, without any magnifying lense between. The image of the sun in this lense, as could be easily seen, would be about and the image of Venus would be about one-thirtieth of an inch. What was peculiar in the American instruments was this : that they not only measured the distance, but also measured the angle of position. They did not fiud the effect simply in the distance of the centres, but also in the direction. He believed no other nation had a similar apparatus. He believed that next to the American system came the French. As far as he had ascertained, he did not believe that the English, Germans, or Russians had photographic instruments as perfect as the Americans and French, He believed, therefore, that each of the American photographs would be superior to each one of the photographs taken by the other nations mentioned. The measure of angular position, however, was peculiar to the Americans. He would u>w tell them how many observations his party had taken. They had got, about 299. [Applause.] From that number, however, would have to be deducted over (50, so that of useful observations there were 237 — 237 measurements of one contact He had to be explicit on this point, in order not to depreciate the observations of his party. They had not relied upon the contacts. He had heard it said, and he had seen it stated in the public Press of New Zealand, that the American party had only got one-half of the observations, because they had observed only one contact. That was quite a different affair. Besides the photographic observations, they had also taken direct observations of Venus, in order to examine especially the physical phenomena of the contact ; and they had, also, at the same time, obtained a series of measurements which would give actually the moment of ingress.