MIC110METRIC SCALES. 



even and uniform that no irregularity in their edges is discover 



able by any microscopic power to which they are submitted. In 



the process of tracing the divisions, the point is moved over the 



glass, the latter being fixed, or the 



glass moved under the point by means 



of a very fine screw, called a micrometer 



screw, the magnitude of the thread of 



which is exactly known. The head 



of this screw is a metallic disc, fig. 3 ; 



the circumference of which is divided 



into from 200 to 400 equal parts, or 



even into a still greater number. 



Let us suppose, then, the screw to be 

 so fine that there are 50 threads to an 

 inch, and the circumference of its head 

 is divided into 100 parts ; one revolu- 

 tion of the head will therefore move the 

 screw and the diamond point upon which it acts through the one- 

 fiftieth part of an inch. But if a fixed index be directed to the 

 circumference of the head, so that the motion of the head through 

 one division can be observed, such motion will move the diamond 

 point through the 1- 5000th of an inch. 



The cutter, after tracing each division, is raised from the glass, 

 while the diamond point is pushed forward by the screw to the posi- 

 tion necessary to engrave the next division of the scale, and proper 

 mechanism limits the motion of the screw, so as to regulate the 

 relative lengths of the divisions of the scale in the manner already 

 explained. 



9. A scale thus engraved, being viewed with a microscope whose 

 magnifying power is proportionate to its minuteness, the divisions 

 are rendered as distinctly visible as those of an ordinary rule are 

 to the naked eye, and if the object to be measured be laid upon the 

 glass its dimensions may be ascertained, as those of an object of 

 ordinary size would be by a common rule. 



10. These scales vary in the magnitude of their divisions, ac- 

 cording to the magnitude of the objects which they are intended 

 to measure. On those which have the largest divisions, an inch 

 is divided into 500 parts ; scales, however, are furnished by the 

 opticians for microscopes in which an inch is divided into 2500 parts. 



1 1 . However minute such scales may seem, they are by no means 

 the most minute that have been executed. Mr. Froment, whose 

 apparatus for the division of astronomical instruments is well 

 known, has supplied me with a scale in which a millimetre is di- 

 vided into 1000 equal parts. Each division of this scale is, 

 therefore, only the 1 -25000th part of an inch. 



