80 LOCAL PARTICULAES OP THE TRAKSIT OF YENFS, 1874. 



the telescope. As the shutter flashes past d e, E E opens every 

 part of it in succession, and exposes the whole sun. In the 

 position in which the shutter is represented, the hole d e is 

 covered, and the shutter must be put into that position before 

 every plate is put in. It is usual to burn the string, but a device 

 such as that in figure 10a will fasten and release it as often as 

 necessary, by just pressing the spring with the fingers at A & B, 

 and it will release the shutter atC The methods of determining 

 the distortion produced in tlie photograph will be found in Mr. 

 De la Rue's and Mr. Rutherford's papers. I ]\a.ve only to say 

 that, by photographing a known pair of double stars such as Alp)ha 

 Centaui'i in all parts of the field, the distortion would be easier 

 and better determined. I shall erect one of the proposed scales 

 about two miles from the Observatory, which will be open to 

 any one who would like to use it (on application). Lastly, and 

 very important, the exact time of each flash of the shutter must 

 be exactly recorded. 



It seems desirable, if possible, to have something which shall 

 form a reliable point of reference daring the transit. For this 

 pui'pose I propose to set up one of De la Eue's scales in such a 

 position as, by simply moving the telescope in right ascension, it 

 may be brought into the field and photographed. If set up truly 

 horizontal, its angle of position could be calculated exactly, and 

 determined by repeated reference from the telescope after star 

 adjustments, and having obtained its true angle of 2:)osition, it 

 Avould form a valuable reference line for the true zero of jDOsition, 

 and at the same time furnish a photo, of the scale as a check on 

 the distortion which might be changed by an alteration of tele- 

 scopic adjustments during the progress of the observations. 



If means are not at command for making the adjustments for 

 teinperature of tube, as suggested by Mr. Rutherford, the tubes 

 may be covered with blankets to prevent change of temperature. 



I hope no one will think that, because a telescope is less than 

 4 inches, it is not therefore to be used. There is no charm in 4^ 

 inches. It is only taken as a size which may be within the reach 

 of a great many observers, and, for the reasons stated, it is 

 desirable (not necessary) that all the telescopes should be as 

 nearly as possible one size. 



I had nearly forgotten one important point, viz., — the method 

 of getting the focus. Mr. Rutherford describes it (and the same 

 plan applies to an ordinary telescope) ; but if the telescope is a 

 good one, much time may be lost, for in some cases the hundredth 

 part of an inch is sufficient to put the telescope in and out 

 of focus. It will greatly facilitate finding it if the plates are 

 put in with a piece of glass of the same thickness, under one end, 

 i.e., out of collimation. One part of the picture will then be 



