Interference Methods to Astronomical Measuremen ts. 19 



removed) the two pencils will meet in the focus in the same 

 phase, and the breadth of the interference-bands may be 

 regulated by slight alterations of the pieces of the refracto- 

 meter, without in any way affecting the intensity of the light. 

 But beside this incidental advantage is the much more 

 important one, that it is possible, by simply increasing the 

 distance between the mirrors a b, to enormously increase the 

 effective aperture of the telescope. 



Plate II. fig. 2 shows, on a scale of one-fourth actual size, 

 a plan of a proposed instrument for effecting this object. 

 The optical arrangement adopted is that shown in fig. 11. 



This form, notwithstanding the greater number of glasses, 

 has some decided advantages over that of fig. 10. In addition 

 to greater facility and ease of adjustment, it has the important 

 advantage of preserving constant the width of the fringes 

 notwithstanding the angular displacement of the whole 

 apparatus *. 



The instrument figured in Plate II. would be used as 

 follows : — The mirrors a and b being moved as close together 

 as possible, and the auxiliary mirror m being in place, the 

 mirror a is adjusted by the screws SS till the two images of 

 a source of light L as viewed in the telescope T appear to 

 coincide. Next the mirror m is adjusted in azimuth by the 

 screw K till the paths of the two pencils are equal. (As 

 before mentioned, this angular motion does not affect the 

 mutual inclination of the pencils, and therefore the breadth 

 of the fringes is unaltered.) The telescope T is then adjusted 

 till the two images of its illuminated cross-hairs coincide with 

 the cross-hairs themselves, and is then clamped. 



The mirror m is now detached and the instrument is ready 

 for use. 



Suppose the object to be measured is a minor planet or 

 satellite. The whole instrument, which would have to be 

 placed on an equatorial mounting, is pointed so that the 

 image of the body is exactly on the cross-hairs. The inter- 

 ference-fringes will at once appear if the adjustment has been 

 properly made. 



Next, by means of a right-and-left-hand screw, the mirrors 

 a and b are separated until the fringes disappear. If this 

 disappearance is due to an accident, it will immediately 

 become evident by observing any star in the neighbourhood. 

 If the examination of the star shows the fringes while they 



* The same advantage obtains in the use of the instrument as a refrac- 

 tomer for measuring extremely small variations of the angular position 

 of the mirror or mirrors m. 



C2 



