CONSTRUCTION OF THE MICROSCOPE. 



27 



doublet or achromatic lens. It is generally set in a folding case, as re- 

 presented in the figure, and so contrived that it is admirably adapted 

 for the waistcoat-pocket ; which, together with the small Jwlder, fig. 22, 

 for securing small objects and holding them during examination, are 

 all that is required for afield instrument during a day's ramble. The 

 useful little holder may be purchased in a case of Mr. Weedon, 41 

 Hart-street, Bloomsbury. The Stanhope lens is similarly constructed, 

 although not so good and convenient as the former, and is but seldom 

 to be purchased properly made. 



When the magnifying power of a lens is considerable, or when its 

 focal length is short, and its proper distance from the object equally 

 short, it then becomes necessary to be placed at a proper distance with 

 great precision ; it cannot therefore be held with sufficient accuracy 

 and steadiness by the unassisted hand, but must be mounted in a frame, 

 having a rack or screw to move it towards or from another frame or 

 stage which holds the object. It is then called a microscope ; and it 

 is furnished, according to circumstances, with lenses and mirrors to 

 collect and reflect the light upon the object, with other conveniences. 



The best of the kind was that contrived by Mr. Ross : it is repre- 



fig. 23. 



fig. 24. 



sented in fig. 23, and consists of a circular foot &, from which rises a 

 short tubular stem d, into which slides another short tube c, carrying 

 at its top a joint /; to this joint is fixed a square tube a, through 



