690 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



describe a curve, and sliding in the two slits, will move the triangular 

 pieces and with, them the tubes forward in a horizontal plane. 



An index z (with a vernier) on the axis A serves to measure the 

 angle made by the revolution of the axis, on a scale graduated from the 



Fig. 80. 



central point of the arc to 90° in either direction, whence the length of 

 the radius can be calculated by appropriate formulae given by the 

 author. 



If however what is required is to determine the distance between the 

 principal points of two reflecting surfaces on the same axis — for instance, 

 to measure the thickness of the cornea — an arrangement is necessary by 

 which the point of convergence can be moved from the principal point 

 in the one surface to that in the other, the common axis of the two sur- 

 faces bisecting the angle between the tubes. The latter must therefore 

 be capable of being moved parallel with this axis without changing their 

 relative position. For this the same eccentric mechanism is used as 

 described above. The plate P is not fixed immovably to H, but a thin 



