CONSTRUCTION OF THE MICROSCOPE. 



35 



securing small objects and holding them during examina- 

 tion, are all that is required for fx field instrument during 

 a day's ramble. This useful little holder may be purchased 

 in a case at Mr. Weedon's, 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, accord- 

 ing to circumstances, with lenses and mirrors to collect 

 and reflect the light upon the object, with other conve- 

 niences. » 



Fig. 23. — Ross's Simple Microscope. 



The best of the kind was that contrived by Mr. Ross, 

 represented in fig. 23 ; and consists of a circular foot e, 

 from which rises a short tubular stem d, into which 



d2 



