MICROMETIIV. 345 



thirty of these divisions, or in otlier words is o"326 mm. in that 

 dimension. It is obvious that this vahie for the divisions of the 

 eye-piece micrometer is good only while the conditions remain 

 the same. 



The form of micrometer eye-piece devised by Jackson has 

 been largely replaced by one in which the scale, an arbitrary one, 

 on a circular glass is fixed at the focus of an ordinary negative 

 eye-piece. The micrometer eye-pieces with movable scales are 

 represented by the cobweb micrometer invented by Ramsden, and 

 first used for telescopes. Fig. 7 shows the eye-piece, and Fig. 8 

 the cobwebs and comb. 



It consists of a positive eye-piece, in the focus of which two 

 fine parallel wires or cobwebs are stretched across the field. One 

 of these wires is fixed at the centre of the field, while the other 

 can be moved by means of a screw, having fifty or a hundred 

 threads to the inch, and whose outer end is attached to a gradu- 

 ated disc, usually divided into a hundred parts. On the lower 

 side of the field is a metal comb having notches which correspond 

 with the threads of the screw. This comb serves to register entire 

 revolutions of the screw, while fractional parts are indicated by the 

 disc. It is necessary to ascertain the value of each revolution or 

 part of a revolution of the screw in the manner already described. 

 The objection to this micrometer is that the measurement of an 

 object cannot be made in the centre of the field. 



Among other modifications of this micrometer is one by 

 Nelson, in which the fixed wire was placed five notches from the 

 centre of the field on the side furthest from the screw head ; and 

 later the same microscopist still further improved it so that both 

 wires could be moved en bloc across the field, and also by adopting 

 a compensating eye-piece. 



Zeiss has devised a screw micrometer eye-piece for objects 

 which occupy a large part of the field. It consists of a Ramsden 

 eye-piece having a glass-plate with crossed lines, and the whole 

 can be moved across the field by means of the micrometer screw. 

 A compensating eye-piece can be employed instead of the Rams- 

 den for use with apochromatic lenses. 



The ease and accuracy of measuring objects by means of 

 photomicrography is familiar to all who have had experience with 



International Journal of Microscopy and Natural Science. 



Third Series. Vol. VI. x 



