MICROMETRY. 



343 



divided into twenty equal parts, so that each division of the scale 

 indicated i/ioooth of an inch. The small prism, a, served to 

 register the number of revolutions. Figs. 3, A, B, and C, show a 

 sectoral scale used upon the stage to determine the actual value 

 of the revolutions of the screw. It was two inches long, and 

 i/ioth of an inch at the base. Hartnack devised a diagonal 

 scale on the same principle, which consisted of fifty vertical lines 

 crossed by two horizontal lines, whose distance apart equalled five 

 divisions of the vertical scale, and which was crossed by a diagonal. 

 Thus if the length of the vertical line was o'5 inch, the shortest 

 line^on the lower side of the diagonal would be o'oi inch. 



Micrometers may be divided into two general classes : — First, 

 stage micrometers, in which both object and micrometer are mag- 

 nified together : second, eye-piece micrometers, in which the scale 

 is applied to the magnified image of the object. In the latter class 

 the scale, or micrometer proper, is usually made a part of the eye- 

 piece, so that the whole becomes a micrometer eye-piece. 



Stage Micro meters. — ^These in their crudest form are represented 

 by the sand grains of Leeuwenhoek, and the silver wires of Jurin. 

 The micrometers of Coventry were made of glass, metal, ivory, 

 etc., and bore lines ruled with a diamond at some uniform distance 

 apart. The use of these micrometers for the direct measurement 

 of microscopic objects was, of necessity, very limited, being con- 

 fined to low powers where the object and scale could be brought 

 practically to the same focus. In other ways, however, these 

 micrometers are very important. Their use with the camera 

 lucida and in standardising eye-piece micrometers is familiar to 

 all microscopists. In the former case any object drawn can be 

 measured by replacing the object by the micrometer, the conditions 

 remaining the same. If the size of the object drawn is arrived at 

 by dividing the size of the image by the magnifying power of the 

 microscope, the distance from the eye-piece to the surface receiv- 

 ing the image must be ten inches. 



More recently Zeiss has offered a stage screw micrometer, 

 shown in Fig. 4. This is intended for the accurate measurement 

 of objects too large for one visual field of the microscope, and can 

 be adapted to the stage of any of the larger stands. The plan of 

 this micrometer is sufficiently evident from the illustration. The 



