1894.] on the Astronomical Telescope. 307 



In the photographic method, however, the record of the observa- 

 tion is not that of any one moment, it is the aggregate of all the 

 impressions made every second and every part of a second during the 

 exposure. The photographic plate, unlike the eye, takes note of, 

 and records every position of the star image, and not the one selected 

 position as the eye does ; hence you can easily see the great necessity 

 of having the utmost possible perfection in the clock driving 

 arrangements. 



This condition of perfection is popularly supposed to be satisfied 

 by having a perfect clock, but there is hardly a portion of the 

 instrument that can be mentioned, the perfection of which does not 

 contribute in some way or other to the accuracy of the motion. 



1. The instrument must have a stability far beyond what is 

 necessary for ordinary work, otherwise the very handling of the slow 

 motions will sensibly affect the positions of the images and injure 

 the results. 



2. It is evident that the axis on which the instrument revolves 

 must be of extreme accuracy, otherwise the instrument will not move 

 truly. 



3. The anti-friction arrangements must be of the most efficient 

 nature in order to give the clock a fair chance of doing its work. 



4. The slow motions must be extremely perfect, as otherwise it 

 will be impossible to bring the guiding star on the cross wires of the 

 " guider " with that accuracy necessary for the best results. 



5. The arrangements, even for the adjustment of the instrument, 

 so far at least as the placing of the polar axis parallel to the pole of 

 the earth, must be such as to enable the observer to make this adjust- 

 ment with an accuracy not at all necessary for visual observations. 



Professor Eambaut, Royal Astronomer of Ireland, has recently 

 shown that whereas for ordinary visual work it suffices if the polar 

 axis of a telescope be adjusted in altitude and azimuth with an 

 accuracy of 1 minute of arc, errors of a few seconds of arc only are 

 allowable with a photographic telescope, and that this great degree of 

 accuracy is best obtained from measurements of the photographic 

 plate itself. 



An instrument, therefore, that is required to give the best results 

 used photographically, should be made with a view to such work in 

 all its details from the very beginning, for an instrument that may 

 be excellent for ordinary observations will most probably break 

 down under the stringent conditions necessary for the more modern 

 work. 



In the usual form of mounting it is necessary to reverse the 

 instrument when observations have been made on a star as it passes 

 from the east to the west of the meridian. This is a great disad- 

 vantage in photographic work, and in designing the new 26-inch 

 photographic telescope, which Sir Henry Thompson has undertaken 

 to present to Greenwich, I have arranged to allow complete circum- 

 polar motion so that this reversing on the meridian will not be 



t 2 



