the Index oj Befr action. 109 



deviation^ d L, of the ray La be refracted by the prism, may 

 be ascertained by mounting the prism with its glasses in front 

 of the object-glass of the telescope of a theodolite, or other 

 angular instrument, as represented in the figures. The prism 

 being so placed as only to cover part of the object-glass, one 

 half is appropriated to the reception of the refracted, and 

 the other to that of the reflected rays ; and the telescope 

 being turned from the object, until the images produced by 

 the refracted and reflected rays appear successively in the field, 

 the corresponding deviations are easily ascertained. 



If the surfaces of the prism are sufliciently flat and well 

 polished to afford a distinct reflected image, a better arrange- 

 ment for obtaining the angle of the prism, which dispenses 

 with the use of the glass D C, is represented in figs. 4 and 5. 



The apparatus by which the prism is mounted before the 

 telescope, is represented in fig. 6. R is a collar fitted to the 

 end of the telescope T U, to which it may be fastened by the 

 clamping-screw T. From opposite extremities of the diame- 

 ter of this collar, two arms N N' project in the direction of the 

 axis of the telescope, which are furnished at their extremities 

 with the screws MM', whose points are presented to each 

 other. By means of the motion which the collar R has on the 

 telescope, and by varying the length of the arm N', which 

 consists of two pieces fastened together with screws S, tapped 

 into one piece, and moving in an elongated aperture in the 

 other, the line joining the points of the screws M M', may be 

 made perpendicular to the limb of the instrument on which 

 the angles are to be measured. The screws M M' carry, be- 

 tween their points, a ring G H, whose plane is therefore per- 

 pendicular to the limb of the instrument ; and which revolves 

 freely round the line joining the points of the screws. The 

 object of this motion is to adjust the prism to its position of 

 minimum deviation ; and for very nice observations, a slow 

 motion may be given to the ring by the wheel and pinion W V, 

 but this may be dispensed with in ordinary cases. Another 

 ring E F, furnished with a piece of perfectly parallel plate- 

 glass A B, is fitted into the ring G H, so as to revolve about 



