254 THE APPLICATIONS OF PHYSICAL FORCES. [BOOK in. 



in a word, they were not dispersed. Since that time the combinations 



necessary to give achromatic systems have been very varied. 



In every carefully-constructed instrument this defect of chromatic 



aberration is suppressed or at least considerably lessened. 



In Galileo's telescope achromatism results 

 partly from the circumstance that the eye-piece 

 is a divergent whilst the object-glass is a con- 

 vergent lens. By taking care to make the eye- 

 piece of flint glass and the object-glass of crown, 

 the system will be achromatic ; but in this case 

 the curves of the lenses would only give a very 

 slight magnification, insufficient for general use. 

 Lenses are therefore preferred in which achro- 

 matism is obtained separately. 



Fig 191 represents an opera-glass, in which 

 the combinations adopted for the eye-piece and 



FIG. loi opera glass with object-glass may be' seen. The latter is formed of 



achromatic object-glass ' . . , , ' 



and eye-piece. a bi-concave flint glass lens inclosed between two 



convex lenses of crown glass, whilst the eye-piece 



is a convex lens of flint glass placed between two concave lenses 



of crown. Sometimes the object glass alone is achromatic and the 



FIG. 192. Double or binocular opera-gloss. 



curve of the eye-piece is calculated in order to increase the magni- 

 fying power. 



