1916] on The Movements of the Earth's Pole 681 



vertical ; it is intended to be used only in the vertical position, and 

 the angle covered by the photog^raphic plate will be a few degrees 

 from the zenith on each side. Exactly how far we can go from the 

 zenith depends upon the qualities of the lens, and no confident 

 statement can be made until this has been tested, but it is hoped 

 that star trails perfectly sharp for measurement will be secured up 

 to an augular distance of 3° from the centre. This gives us as 

 available for our purpose the stars over a belt 6° wide down to the 

 sixth and possibly the seventh magnitude. The actual work of 

 observing will be very simple, and will only mean that the whole 

 instrument is rotated through 1<S0^ at certain prearranged times, and 

 that the lens is opened after twilight and covered before the dawn. 

 It would be possible for this to be done by mechanism controlled 

 by a clock. 



As the telescope hangs freely always in a vertical position, we 

 entirely get rid of one of the astronomer's anxieties, the risk of error 

 due to flexure or bending of his telescope, for though the tube can be 

 made apparently very rigid, the excessively minute degree of bending 

 sufficient to introduce appreciable errors is difficult, if not impossible, 

 to avoid in a telescope which has to be used in different positions. 

 Then, again, the errors due to changes of temperature inside or close 

 to the instrument should almost disappear in this form. Firstly, no 

 temperature changes affect the suspension ; so long as the body of the 

 telescope remains undistorted the position of the true vertical in 

 regard to the optical axis remains constant. Secondly, as the whole 

 hanging part of the instrument is perfectly symmetrical about the 

 vertical axis, with the trifling exception that the plate-carrier and 

 photographic plate are not circular but rectangular, no temperature 

 change should distort the axis. Any distortion that can take place 

 will, in fact, be the very small change of scale that will result from 

 the difference in the expansion of the glass plate and the brass tube. 

 Thirdly, it is possible, and in this instrument has been done, to 

 enclose the whole in an outer case which can be made air-tight and 

 kept at a constant temperature by a thermostat. In order to close 

 the instrument in front it is necessary to have a plane parallel glass of 

 slightly larger aperture than the lens. As this glass has to be worked 

 with the same refinement as a lens, and as a plane surface is more 

 troublesome to work than a curved one, this is rather a costly addition. 

 Whether, as a matter of fact, it is worth while keeping the instrument 

 at the same temperature, or whether it will be better to reduce the 

 temperature change to a minimum by covering the whole with non- 

 conducting material, and then apply the very smaU corrections 

 necessary to the measurements made on the plate, is a question for 

 experience to decide. 



As a heavy hanging mass would be liable to long-continued vibra- 

 tions when disturbed, a four-armed vane attached to a rod at the 

 base is immersed in a dash-pot or bath of glycerine. This rod 



