116 



geneity to make low powers, and their cost would 

 be too considerable, without a corresponding ad- 

 vantage. They are usually only made in powers- 

 of ^ inch and stronger and for this reason have 

 but a limited use. 



Ramsden Eyepiece. This is a positive eye- 

 piece and is constructed of two plane convex lenses 

 with the convex surfaces toward one another. 

 They are especially useful as micrometer eyepieces 

 in microscopes and telescopes, as they are used as 

 a magnifier on the fine divisions of micrometer and 

 at the same time give the virtual image. 



Periscopic, Orthoscopic and Kellner Eye- 

 pieces. These are also positive eyepieces which. 



c 



Fig. 38. 



are achromatized by making the eye lens a doublet, 

 or triplet and by its correction permits the use of 

 a larger and flatter field. (Fig. 38.) 



