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A. M. Mayer — WeU-Spherometer. 



the plate closing the back of the well. Hinged to the top of 

 the back of the well is a mirror ra, and this is pressed against 

 the point of the rod r, by a delicate helical spring. When the 

 point of the rod r, touches a plane which closes the mouth of 



the well, the plane of the 

 mirror is vertical. A beam 

 of light from o, falls on the 

 mirror and in the present posi- 

 tion of the latter the beam 

 X is reflected back on o. By 



N n placing two or three plates of 



s s x known thickness between the 



plane covering the mouth of 

 *\, s the well and the point of 



the rod r, the beam m A, is 



reflected upward on a vertical scale A, B. The linear values 

 on this scale of equal parts may be determined from these 

 measures. The tilt of the mirror can be measured by observ- 

 ing in a telescope, placed on the line o m, the reflection of the 

 scale from the mirror m. The amount of motion of the rod 

 against the mirror is as the tangent of the angle of rotation of 

 the mirror, and the reflected beam moves over a space equal to 

 twice its horizontal distance from the screen into the tangent of 

 angle of tilt of the mirror. It is evident that the height of the 

 section of the lens, L, contained within the mouth of the well 

 can thus be measured. 



