MEASUREMENT OF OBJECTS. 



thickness of the glass which covers the scale ; and however thin 

 this glass may he, where high powers are used, it will render the 

 difference of the foci of the scale and the ohject so sensible, that 

 they can never be both seen with sufficient distinctness at the 

 same time. 



57. We know no greater example of the inexhaustible resources 

 of art, and the untiring zeal with which its cultivators minister to 

 the wants of science, than the wonderful perfection to which the 

 mechanical division of a material so fragile as glass has been 

 carried. For the reasons we have here stated, as well as because 

 in the application of the highest magnifying powers the object- 

 glass of a microscope requires to be almost in contact with the 

 object, without actually touching it, microscopists required 

 extremely thin plates of glass to cover delicate objects mounted 

 on their slides. Messrs. Chance of Birmingham responded to this 

 demand by the production of plates of glass so thin, that three 

 hundred of them piled one upon the other are no higher than 

 an inch. 



For examples still more striking of the minuteness with which 

 lines may be traced upon glass by mere mechanical processes, we 

 may refer the reader to that part of our Tract upon Microscopic 

 Drawing and Engraving, in which the test plates of Mr. Nobert 

 are described. 



58. One of the most evident expedients for the measurement of 

 microscopic objects would seem to be the micrometer screw, which 

 is applied with so much success, and with results of such extreme 

 precision, in astronomical instruments. Various methods of apply- 

 ing it to the microscope will suggest themselves to every one who is 

 familiar with its uses in the observatory. Let two filaments of 

 spider's web, or micrometric wire, be extended at right angles 

 across the field in the focus of the Fig. 25. 

 eye-piece. These will divide the field 



horizontally and vertically at right 

 angles, intersecting at its centre, as 

 shown in fig. 25. Now suppose 

 the stage supporting the object is 

 capable of being moved by a micro- 

 meter screw, having for example 

 one hundred threads to the inch. 

 Let the object be placed first so that 

 its length shall be horizontal, and 

 let the slip be adjusted so that 

 the vertical micrometric wire b V 

 shall coincide with one of its extremities. Let the micrometer 

 screw be now turned so that the object shall move horizon - 



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