216 USE or THE MICROSCOPE. 



The cobweb micrometer, the invention of Kamsden, is 

 represented by fig. 143; it is Composed of a positive eye- 

 piece (fig. 138), in the focus of which two very fine parallel 

 wires, or cobwebs, are stretched across the field ; one of these 

 wires can be separated from the other by a screw, commonly 

 provided with a hundred threads to the inch, and the head of 

 of which, as shown by fig. 143, is also divided into a hundred 

 parts. The field of view, which is represented by fig. 144, is 

 made flat on its lower border, by means of a comb, made of 

 a thin piece of brass, whose edge is indented with notches, 

 made by the threads of the same screw; every fifth notch is 

 longer than the others, to facilitate counting, and each notch 

 corresponds to one turn of the milled head, so that the nmn- 

 ber of turns can be read off" in the field of the instrument, and 

 the fraction of a turn on the divided head. 



Thus, by this simple contrivance, the distance of the wires 

 can be ascertained to the hundredth of the turn of the screw, 

 and, as it has been stated that the screw has a hundred 

 threads to the inch, it follows that the magnified image of an 

 object may be measured to the ten-thousandth of an inch; 

 but with an object-glass of one-eighth of an inch focus, the 

 image wiU at least be magnified eighty times, without the 

 power of the eye-piece ; it follows, therefore, that a quantity 

 as small as the eight-hundred-thousandth of an inch should be 

 appreciable by such an instrument, but in practice this has 

 been found impossible, as no achromatic power has yet been 

 made, capable of separating lines that are closer together than 

 the one seventy-five-thousandth of an inch. This micrometer, 

 when well made, is rather expensive, and requires some con- 

 siderable care in using ; and as its accuracy depends entirely 

 on that of the glass micrometer used in finding the value of its 

 divisions, the measurements made by it are by no means so 

 dehcate as they appear to be. 



