THE BEST FORM OF MICROMETER EYE-PIECE 273 



one, a flat rectangular box containing the fixed and movable webs, 

 the micrometer screw, and divided head complete ; the other part 

 may be called an ' eye-piece adapter,' with an outer case to hold the 

 above-mentioned rectangular box. 



The flat inner box has a screw attached to it which engages with 

 a head on the exterior of the outer box. This gives about one inch 

 of screw movement to the inner box, which causes the webs to 

 traverse the field of the microscope. It must be remembered that 

 this in no way affects the movement of the movable web from the 

 fixed, which can alone be accomplished by turning the graduated 

 micrometer head as in the old form. 



The ' eye-piece adapter ' portion of the instrument is, as its name 

 implies, merely an adapter to take the optical part of positive com- 

 pensating eye-pieces of various powers. 



Immediately below the web itfaji iris diaphragm. This permits 

 a diaphragm to be used suitable to the power of the eye-piece 

 employed. A guiding line at right angles to the webs has been 

 added. Care must be taken to observe that when the movable web 

 coincides precisely with the 

 fixed web, the indicator on the 

 graduated head stands at zero. 

 If this is not the case, the 

 finger screw must be loosed, 

 which will liberate the gradu- 

 ated head, and then it can be 

 placed in its proper position 

 and fixed. This is of universal 

 application to all screw micro- 

 meters. 



Four points are gained by _ __ 



this arrangement :- FlG 214 ._ Nelson . s new form of 8Crew 



(1) The compensating eye- micrometer eye-piece, 

 piece yields far better defini- 

 tion when measuring with apochromatic objectives than either the 

 Huyghenian or Ramsden forms. 



(2) Different-powered eye-pieces can be employed. 



(3) By means of the screw which moves the micrometer webs 

 across the field it is possible to perform measurements with the webs 

 equidistant from the centre of the field, and thus eliminate errors 

 due to distortion. 



(4) The preceding advantage is secured without sacrificing the 

 benefit of a fixed zero web. 



Messrs. Zeiss have since .adapted the compensating eye-piece to 

 their best screw micrometer. 



To use the screw micrometer vnik success it should not be inserted, 

 as the custom has been, like an ordinary eye-piece into the tube of 

 the microscope, but it should have a form stand quite independently, 

 preventing actual contact with the body- tube. 



Plate II. gives the mode of its employment, the illustration being 

 made from a photograph by Mr. Nelson. The micrometer eye-piece, 

 it will be seen, is fitted into a stand wholly independent of the 



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