HOW TO WATCH THE TRANSIT OF VENUS.

Globe, Volume II, Issue 105, 1 October 1874, Page 3

HOW TO WATCH THE TRANSIT OF VENUS.

In answer to the queries of one or two correspondents who are desirous of learning, a simple method of watching the phenomenon of the transit of Venus across the sun’s disc, Mr Ellery, Government astronomer, has kindly supplied the following notes to the Australasian, which will be useful for the guidance of amateur observers Observations of the transit of Venus across the sun’s disc will be principally confined to the determination of the exact time at which the planet’s disc enters upon that of the sun ; and those particular instants when the outer edge of the planet is in precise coincidence with the sun’s edge, called internal contacts, will be taken. These contacts occur when Venus first shows as a full round disc on entering on the sun’s disc, and again while she still shows a round disc just before egress. Such observations require the use of good telescopes well mounted, with apertures of not less than Sin or 2Jin; they also require the use of a good clock or chronometer, and the exact local mean time or sidereal time. For simply viewing the phenomenon very simple apparatus will be sufficient. To good eyes protected by deep colored or smoked glass, Venus will be visible after she is well on the sun’s disc, Any ordinary telescope, and even

binocular, opera or field glasses, will give a better view; but whatever telescope is used, great care is necessary to protect the eye from the intense glare and heat concentrated at the eye aperture, and if proper sunshades are not provided with the telescopes, they must be improvised before attempting to look at the sun, for many have lost or injured their eyesight by not attendin'! to this most necessary precaution. Deeply colored glass, especially a piece of deep red, and a piece of deep green or blue together, would be found very safe and pleasant ; a piece of blue and green, or deep neutral tint, will also do nicely. If none of these can be obtained, the old-fashioned smoked glass will form an excellent substitute. The proper method of making this is : Obtain two strips of thin clear glass, say Sin long by 1 in broad (the glass sold for microscopic slides is the best); around the margin of one of the surfaces paste a strip of thick writing paper about the eighth of an in h broad, and smoko this surface by holding the slip over a candle with a long wick or small kerosene flame (without chimney), taking care to warm the glass gradually at first, or it will crick. The proper density of the blackening can be got after a few trials. After the rightkindof smoked surface has been obtained the second glass can be fastened over it; the strips of paper will keep it from coming in contact ; by pasting a strip of thin paper over the edges so as to hold the two glasses together a dark shade for observing the sun is formed, which will last any length of time.”