Additions to the Kew Magnetometer. 123 



and " back M views, in which the observer looks towards the 

 sun and stands with his back to it respectively. We have 

 thus been able to eliminate the errors of adjustment of the 

 mirror, and to assure ourselves that each observation was 

 satisfactory. 



This method is, however, open to some objection. When 

 the sky is cloudy it may be impossible to see the sun for a 

 time sufficient to take all four sots of observations. On such 

 occasions we always pay special attention to the adjustment 

 of the mirror, but it may happen that no object near the 

 station is sufficiently elevated to be used for the purposes of 

 correction. In that case it is necessary to rely on the result 

 of a long experience, which proves that if reasonable care is 

 taken of the instrument the mirror does not get seriously out 

 of adjustment during a journey by road or rail extending over 

 a day or two. 



Nevertheless, there can be no doubt that the method of 

 correcting by the image of the cross wires has the advantage 

 that it can, theoretically at all events, be carried out at the 

 place and time at which the observations are made. 



The practical drawback which led us to abandon it was the 

 difficulty of seeing the image of the wires. In the neighbour- 

 hood of the cross wires the metal of the telescope tube is cut 

 away, but the light thus admitted is insufficient. On a 

 gloomy day, when the sun was only visible at rare intervals, 

 and when, therefore, it was most important that the mirror 

 should be in order, the necessary adjustments could not be 

 made. Even on a bright day it was desirable to envelop the 

 observer's head and the instrument in a dark cloth, and this 

 was difiicult and troublesome if the wind was high. 



Wo have, therefore, lately devised some additions to the 

 magnetometer which, although extremely simple, make it 

 practically possible to use the cross wires for making the 

 adjustments. 



A polished platinum mirroris in- l r ig. 1. 



troduced into the telescope between 

 the eye-piece and the cross wires, 

 which are viewed through a hole 

 in its centre. The mirror is in- 

 clined at 45° to the axis of the tele- 

 scope, and reflects a considerable 

 quantity of light upon the wires. 

 A bronzed brass tube is attached to a support which fits into 

 the three holes which arc provided to fix the magnet-box 

 in position. This tube extends from the telescope to the 

 mirror, and thus shut.- out i : ^ht. It can be 



