THE OPTICAL ARRANGEMENT. 



261 



5. THE EXTENT OF THE FIELD OF VIEW. 



The extent of the field of view in a given system of lenses is 

 dependent upon the aperture of the lenses and the position of the 

 observing eye. The greater the distance of the latter, the smaller 

 is the field of view. If again JV (Fig. 141) is the first, and 



FIG. 141. 



N' the last refracting surface of the system, whose principal and 

 focal planes are denoted in the usual way, and if its aperture, is 

 determined by the diaphragms or the settings B B, the eye situated 

 at o views the object a b in its whole extent, because all pencils 

 incident parallel with the axis are refracted towards o. But if 

 the eye is placed at o', then only those pencils can reach it which 

 diverge so that their point of convergence p is in the same relation 

 to o' as an object to its image. The field of view becomes therefore 

 necessarily less than the aperture of the system; the outermost 

 points still perceptible are of and V. A withdrawal of the eye to 

 a still greater distance involves a stronger divergence of the 

 incident pencils, and therefore a further diminution of the field, 

 and vice versa. It is obvious that the field of view is dependent 

 upon the aperture of the lenses, and would be reduced by 

 smaller diaphragms. 



B. The Optical Arrangement. 



We will now pass to the description of the more popular forms 

 which, in recent times, have been given to the magnify ing-glass and 



