of Methods for Observing Small Rotations. 



97 



its edge just clears the incident beam. If the distance between 

 the surfaces is equal to the diameter of the moving mirror 



Ffc. 6. 



M' 



the angle of incidence will be about 20° and the aperture 

 effective for resolution will therefore be about 92 per cent, of 

 the full aperture. If we need to read deflexions on both 

 sides it will be necessary to place the second mirror m' so 

 that it makes a larger angle with the first, as in fig. 6. In 

 this case the effective aperture will be less, but may still be 

 made as large as 75 per cent, of the full aperture*. 



It is very easy to add the additional mirror to a magneto- 

 meter or electrometer, or to a galvanometer in which the 

 mirror is between the coils instead of inside of one, as in the 

 older forms of the Kelvin instruments. The doubling of the 

 deflexion for a given movement of the suspended system is 

 particularly advantageous in instruments intended for auto- 

 matic registration (photographic or otherwise), because in 

 such instruments the requisite linear movement on the record 

 sheet can be secured when the distance of the latter from the 

 instrument is only half what is necessary with the customary 

 mirror arrangement. 



Yerkes Observatory, University of Chicago, 

 March 1897. 



* When the angular deflexion is large there is a further slight dimi- 

 nution of aperture due to the lateral displacement of the beam at the 

 second reflexion. When the distance between the mirrors is equal to 

 the diameter of m this lateral displacement is about 2 per cent, of the 

 aperture for each degree of deflexion of the image. 



Eeeata in my preceding article " On the Resolving Power of Telescopes 

 and Spectroscopes for Lines of Finite Width," in the May number 

 of this Journal. 



Page 328, line 5, for maximum read minimum. 

 „ 341, „ 17, for brought read bright. 



„ 341, ,, 20, after telescope-lens insert of rectangular aperture. 

 „ 342, Table V., 4th column, in the heading, for 48 read 4*8. 



Phil. Mag. S. 5. Vol. 44. No. 266. July 1897. II 



