the Index of Refraction. Ill 



the arm N'. The latter adjustment is rendered permanent by 

 tightening the screws S. If the apparatus be in good ad- 

 justment, the reflected image of the luminous object should 

 remain on the horizontal wire, at whatever angle the tele- 

 scope is inclined to the object ; and the process of adjustment 

 ought to be repeated until this is the case. It will afterwards 

 be generally sufficient to adjust the instrument by means of 

 the collar R. 



The apparatus being adjusted, the principal section of the 

 prism, and consequently the plane of reflexion between the 

 glasses, is brought into coincidence with the plane of the di- 

 vided limb by means of the ring E F. The refracted image 

 will now be upon the horizontal wire of the telescope ; and by 

 moving the ring G H, and following the motion of the refracted 

 image, a point will be found where it stops and begins to move 

 in the opposite direction. By means of the tangent screw, 

 the cross-wires are made to intersect the image at this point, 

 at which the deviation is a minimum, and the angle is read 

 ofl". The telescope is then turned round, until the reflected 

 image intersects the cross-wires, and the angle is read off. 

 The direct bearing of the object being then taken, the difl^e- 

 rences between it and the other observed angles will give the 

 deviations of the reflected and refracted rays, which must be 

 corrected for the parallax due to the distance of the prism 

 from the centre of the instrument.* 



From these angles the index of refraction is easily calcu- 

 lated. 



Let 5= the observed deviation of the refracted rays ; 

 ^=the angle of the prism ; and 

 ^=the observed deviation of the reflected rays. 



Then, since the prism is in its position of minimum devia- 

 tion, /A = !^I^AliL_) ; which expressed in terms of the observed 



angles gives /o. = — . ^ 7 * 



* If the distance of the observed object be great, the correction, in seconds, to be 



added to an observed angle « will be nearly ^. — p, ; vrhere d is the distance of 



the observed object, and r the distance of the prism from the centre of the theo- 

 doli e. 



