84 LOCAL PARTICTLARS OP THE TRAXSTT OP TEKUS, 1874. 



the sun on wliicli Venus will a,2:»pear to an observer Avithout a 

 telescope at ingress ; for cards at egress Venus is shewn betvA^een 

 ]N[ and W., bat cards will be printed for each position separately. 



Time. 



The question of time now has to be considered, and it is one of 

 the most important, for without an exact record of the time of 

 each observation the most perfect measures or photos, will be 

 worthless. Where a chronograj^h can be had it should be used, 

 and means taken not only to mark on the tape the dot which 

 represents the time, but also to write opposite each observation 

 or dot what it represents, so that there may be no possibility of 

 subsequent confusion. In the absence of a chronograph, the 

 observer will note the ticks of the clock or chronometer and 

 record the time accordingly ; but it is to be remembered that the 

 rates of clocks and chronometers must be steady, and that nothing 

 but the transit instrument will -determine their errors with the 

 requisite accuracy. Of course, should the observer be on any 

 telegraph line he may get time from the Observatory ; and I 

 shall be only too glad to furnish any aid that may be in my 

 power. 



The exact latitude and longitude of the observer must be known, 

 if he obtains good observations ; but as that can only be known 

 after the transit, the determination of true position may be left 

 till then. 



{Ex-tract>!.'\ 

 Warrex De La Rue, Esq. 



At tlie request of tlie Astronomer Eoyal, Mr. De La Rue has furnislicd a 

 paper on tlie Obserration of the Transit of "\^enus by Photography, from 

 M-hic-h we extract the following notes : — 



" I do not suggest that photographic observations should displace eye 

 observations ; on the contrary, I think that both eye and photographic 

 observations ought to be made. Tlie conditions which the transit of Venus 

 offer for the determination of the relative position of the sun and planet's 

 centres are more advantageous tlian those presented by solar eclipses ; inas- 

 much that it is far more easy to measure directly the distance between the 

 centre of the disk of the sun and that of the image of the planet upon it, than 

 to measure the distances between peripheries of the sun and moon ; and in 

 the transits of Venus any error of obseiTation would not affect the final 

 result nearly so much as in solar eclipses. For example, an error of 1" in the 

 measurements of maxinnim displacement in 1874 or 1882, would give an error 

 of only 0"185 in the deduced solar parallax. Moreover, it is by no meaiis 

 important to catch exactly the phases of contact, as two photographs obtained 

 at a sufficient interval will serve the purpose. Nor is it in any way essential, 



