DR. WOLLASTON'S DESCRIPTION OF A MICROSCOPIC DOUBLET. 13 



end of the tube is closed by a piece having a screw, by means of which it 

 may be fixed in the top of the box intended to contain the instrument, which 

 thus foi-ms a support. A portion of the tube above this piece is cut away, as 

 marked by the dotted line, for the purpose of admitting light to the small 

 mirror which is attached to an horizontal axis passing through the diameter 

 of the tube. The inclination of this mirror may be varied by means of a milled 

 head fixed to the axis on the outside of the tube ; the other adjustment at right 

 angles being made by turning the box of the microscope. 



Into the tube above the opening a conical piece is soldered, into which is 

 screwed a small cylindrical tube carrying the perforation before described. 

 The plano-convex lens is fixed in a spring tube, which slides into that which 

 forms the body of the microscope. The position, consequently, of the lens 

 may be varied so as to bring the image of the perforation into the same 

 plane with the object to be viewed. A piece of plate glass about two inches 

 square, or less if it be thought more convenient, is attached to the top of the 

 tube, and serves to support a stage having lateral adjustments at right angles 

 to each other. The piece into which the magnifiers fit, may be moved by a 

 rack and pinion, and great care must be taken to arrange this adjustment, so 

 that the magnifier may move precisely in the prolongation of the axis of the 

 tube. The tube is divided into two pieces, of equal lengths, which screw into 

 each other, and which when taken asunder will allow of the whole instrument 

 being packed in a box about four inches square. 



Supposing the plano-convex lens to be placed at its proper distance from 

 the stage, the image of the perforation may be readily brought into the same 

 plane with the object, by fixing temporarily a small wire across the perforation 

 with a bit of wax, viewing any object placed upon a piece of glass upon the 

 stage of the microscope, and varying the distemce of the perforation from the 

 lens by screwing its tube until the image of the wire is seen distinctly at the 

 same time with the object upon the piece of glass. 



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