in the Velocity of Propagation of Light in Water. 273 



these two telescopes, and at a distance of about 8 metres, a screen 

 was placed which carried a moveable vertical slit, behind which 

 the name was afterwards placed. A little on one side of the 

 prism a fixed telescope was placed upon a second wooden sup- 

 port, and so directed that by its means, on the one hand, the 

 edge of the refracting angle of the prism could be fixed verti- 

 cally, and, on the other, the angle of the prism could be deter- 

 mined by reflexion-observations. 



Fig. 2 explains this arrangement. T is the theodolite, U the 

 universal circle, P the prism, F the auxiliary telescope : S de- 

 notes the direction of the slit, and the direction of the point 

 of a distant lightning-conductor. The square border is intended 

 to represent that the entire apparatus rests on one and the same 

 stone basement. The following operations are necessary for the 

 adjustment of the instrument : — 



(1) Fixing the axis of rotation of the theodolite vertical by 

 means of a level. 



(2) Centring and fixing the hollow prism in such a manner 

 that the line of intersection of the two including glass plates is 

 parallel to the axis of revolution, and that the latter passes 

 through the plane which bisects the angle of the prism. The 

 first of these two conditions may be completely satisfied by shift- 

 ing the prism by means of the adjusting screws of the support- 

 ing table until a distant object 0, which lies in the same hori- 

 zontal plane as the prism and the axis of the auxiliary telescope, 

 is so mirrored on both sides of the prism that it appears by 

 both reflexions on the horizontal thread of the auxiliary tele- 

 scope. The second condition is only approximately satisfied. 



(3) Placing the slit at the same height as the prism, and en- 

 suring its verticalness by comparison with a plumbline. 



(4) Fixing the universal circle. 



The slit is first illuminated by as strong a light as possible, 

 and the place is sought where the minimum refraction occurs. 

 The universal circle is brought into this place, and its position is 

 changed until the refracted spectral image of the slit is seen in 

 the telescope. It is certainly somewhat troublesome to find this 

 image at first; but when it is once found, the apparatus can be 

 easily placed at the same height as the slit, and its axis of rota- 

 tion may be made vertical without losing sight of the image. 

 At starting I have often used the device of directing the tele- 

 scope of the universal circle to the prism, bringing alight behind 

 the eyepiece, and then fixing the slit at a distance of about 8 

 metres, when the refracted image of the light was seen. After 

 this, the positions of slit and universal circle were more cor- 

 rectly adjusted. When the axis of the universal circle has been 

 adjusted vertically by the level and the minimum of refraction 



