225 



Note on a New Spherometer. 

 By E. M. Nelson, F.R.M.S. 



(Read October 2\st, 1892J 



This spherometer made by Mr. Curties from my design 

 differs from others because the lens to be measured is placed 

 on it, instead of the spherometer being placed on the lens. 



The usual three points are dispensed with, a ring being sub- 

 stituted, which is cheaper to make as well as more accurate. 



The ring, which is held in a tripod stand, has, passing upw^ards 

 through its centre, a micrometer screw having fifty threads to 

 the inch, and the usual drum-head reading to — ^ — inch. 



To use the instrument the drum-head is first brought to 

 zero by means of adjusting screws and a glass " plane surface." 

 The lens is then substituted for the " plane surface," and when 

 the hemispherical polished steel head of the micrometer screw 

 is just brought into contact with the lens the reading is taken. 



There are three rings of different diameters to suit lenses of 

 various sizes. 



The reading of the drum-head shows the length of the versed 

 Sine V, from which, when the chord C is known, the value of R, 

 the radius, may be found by the following formula : — 



2 V 

 By making C a root, the computation becomes one of mere 

 inspection, as can be taken out of a table of reciprocals. 



Thus if C ■= /"8" = 2-82843 inch, 



2 V. 

 When C = / 2~= 1-41421 inch, 



And if C= v^ - 2 = -447214 inch, 



R=_L/ 20v+-L^ 

 40 V v/ 



These are the diameters of the three rings in the instrument 



before 'you. 



