HOW TO WATCH THE TRANSIT OF VENUS.

Thames Star, Volume VI, Issue 1797, 6 October 1874, Page 3

HOW TO WATCH THE TRANSIT OF VENUS.

In answer to the queries of on* or two* correspondents^ fwho are desirous-of learning a simple method of watching the phenomenon «f the transit of Venus across the sun's disc, Mr. Ellery, Government astronomer, has kindly supplied Ihe following notes, 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 due enters ■upon that of the sun ; and those particu-i lar 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 i telescopes well mounted, with apertures : of not less than 3in. or 2|in.; they also* require the use of a good clock orchro-j nometer, and the exact local mean time or sidereal time. , "For simply viewing the phenomenon very, simple apparatus will be sufficient. To good eyes prottcted by deep coloured or smoked glass, Venus will be visible after she is well on the sun's disc. Any ordinary telescopes, and binocular, opera or field classes, will give a better view ; but,.-whatever telescope is used, great cafe 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 attending to this most necessary precaution. Deeply coloured glass, especially a piece of red, and flr piece of deep green, or blue, together,, would be found very safe and. pleasant ; a piece of blue or. 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 this is:—Obtain two strips of thin clear , glass, say 3in. long by lin, broad (the i^iass sold., for microscopic slides is the TbNrtt); around the margin of one of the \ surfaces paste a strip of thick writing .-u^paper about the eighth of an inch broad, * land smoke this surface by holding the slip over a candle with a long wick or a small kerosene flame (without chimney), taking, care to warm the glass gradually at first, or it will crack. The proper density of the blackening can be got after a few trials. After the right kind of smoked surface has been obtained, the second glass can he fastened over it; the strips of paper will keep it from coming in contact; by pasting a strip of thin pape>£over the edges to hold the two glassro together a dark shade for observ- j ing the sun is formed, which will last any *ength of time.'' —Australasian