MICROMETER. 223 



number of the squares, and dividing it by the value of one 

 square, and the quotient will be the dimensions required. 



To Use the Cobweb Micrometer. — Before an object is mea- 

 sured, it must be brought to the middle of the field, and, 

 after the table has been consulted, which shows the value of 

 each revolution of the screw, and of each division of the 

 wheel aflixed to it, the cobwebs must be examined, in order 

 to see whether they both accurately coincide when the zero 

 point of the graduated wheel is opposite the index. The 

 screw being now turned, until the image of the object is, as it 

 were, enclosed between the cobwebs, the number of turns 

 and parts of a turn are both shown by the indices ; the former 

 by the comb at the lower part of the field ofview, the latter by 

 the division opposite to which the index points. It must, how- 

 ever, be borne in mind, that both with this micrometer and 

 with those of ruled glass several measurements of the same 

 object ought to be made ; and if there should be any difference 

 between them, the mean of the two extremes should be taken 

 as the correct one. The measurements made with the cobweb 

 micrometer are said to excel those of all the other forms of 

 instruments ; and that, with an object-glass of one-eighth of an 

 inch focus, even a quantity as small as the eight-hundred- 

 thousandth of an inch can be appreciated. This is, at least, ten 

 times as dehcate as is required ; for Mr. Ross, in his experi- 

 ments, preliminary to the constructing of his beautiful divid- 

 ing-engine, found that, with the highest magnifying powers, it 

 was impossible to ascertain the position of a line nearer than 

 to the eighty-thousandth of an inch. 



Measurements of an Object made by means of a Stage Micro- 

 meter and a Camera Lucida. — For this very valuable plan, we 

 are indebted to Mr. Lister. By means of the camera lucida, 

 a sketch of the object is first made ; the microscope being 

 fixed in the horizontal position, the object is then removed 

 and a stage micrometer placed in the focus of the object-glass 

 instead ; a sketch of its divisions is also to be made on the 

 same paper as the object itself was sketched on, with aU the 

 optical arrangements of the microscope unaltered. The object, 

 therefore, and the micrometer being both magnified to the 



