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SUMMARY OF CURRENT RESEARCHES RELATING TO 



the middle bracket, or lens rest, and containing one or more notches, 

 intended to engage a pin and hold the target-carriage at a fixed distance 

 when testing prismatic power. The lens rest comprises a ring carrying 

 a glass plate, against which the lens to be tested, marked, etc., is held by 

 hand or otherwise on the optical axis extending from the sight tube to 

 the target centre. Pivoted to the bottom of the middle bracket is an 

 arm holding a little shaft placed at right angles to the vertical plane, 

 through the axis of the instrument. This shaft carries three marking 



Fig. 30. 



pins, which, when the arm is lowered, strike an ink-pad, and, when the 

 arm is raised, stamp the lens at three points in the same straight line 

 coincident with the horizontal major axis of the lens. During the 

 swing of the arm the shaft carrying the ink-pins executes a quarter-turn, 

 under the influence of a cord which passes over a drum on the shaft 

 and is kept taut by a spring. The rotation of the shaft is limited 

 by stop-fingers. The marking pins are yielding, so as to compensate for 

 the amount of projection of different parts of the surface of the lens. 

 Detailed descriptions of the use of the instrument are given. 



