OPTICAL IMAGES. 



lens of crown-glass will have a longer focus than a similar one of 

 flint-glass, since the latter has a greater refracting power than 

 the former. 



39. SPHERICAL ABERRATION. 



In all that has been stated hitherto, it has been assumed that 

 the images produced by lenses are as perfect reproductions of the 

 object as is the image produced by a common looking-glass. 



In practice this conclusion requires considerable qualification. 



In the first place, lenses, of whatever material they may be 

 formed, though very transparent are not absolutely so, and they 

 consequently intercept more or less of the light which falls upon 

 them. The thicker they are the greater is the quantity of light 

 thus intercepted. Sometimes there is a tendency to intercept 

 light of a particular tint of colour. In such cases the brightness 

 of the image is not only deteriorated, but it is falsely coloured, 

 being most tinged with those colours which the material of the 

 lens transmits most freely. 



Although such imperfections cannot be totally removed, they 

 may be and have been reduced to so very inconsiderable an 

 amount by the proper selection and adaptation of the material of 

 which lenses are formed, that they need not be farther noticed 

 here. 



The loss of light by reflectors, however highly polished the 

 reflecting surface may be, greatly exceeds the amount of light 

 intercepted by transparent media. On this, as well as some 

 other accounts, refracting have been generally preferred to 

 reflecting microscopes. 



40. Although the image of an object produced by a convex 

 lens in the manner already described (29), appears at first view 

 to be an exact reproduction of the object, it is found, when 

 submitted to rigorous examination, to be more or less confused 

 and indistinct. This confusion is augmented in proportion as it 

 is more magnified, and when it is viewed as in a compound 

 microscope, with a simple microscope so as to be still further 

 amplified, the confusion becomes so great as to deprive the 

 observation of all utility. 



This indistinctness and confusion arises from two causes, one 

 depending on the form, and the other on the material of the 

 lens. 



That which depends on the form of the lens we shall now 

 explain. 



41. If a convex lens be presented to a visible object, the central 

 part being covered by a disc of card, leaving uncovered a ring 

 of surface at the borders, a distinct, but very faintly illuminated 



102 



