PROPERTIES OF LENSES. 13 



shape are formed from different kinds, chromatic 

 aberration will be destroyed. If the lens A (Fig. 10) 

 of high dispersive power, concave form, and low cur- 

 vature, be joined to B of lower dispersive power and 

 greater curvature, it is evident that the dispersion 

 caused by the latter is neutralysed, whilst its refrac- 

 tive power is only decreased by the opposite refrac- 

 tion of the concave. 



The convex lens of crown is generally corrected by 

 a concave of flint, it has been found that no two 

 lenses can be made to correct each other perfectly, 

 so opticians overcome the difficulty by combining 

 several. 



Before leaving this subject we should mention that 

 in photography errors frequently arise, especially 

 when using low powers, owing to the difference be- 

 tween the visual and chemical foci. A point of 

 white light may appear perfectly distinct at E 

 Fig. 9, when surrounded with a violet areola, and if 

 a photograph be taken it will be quite indistinct; 

 while at V nearer the lens, the image may not ap- 

 pear so distinct, but will be surrounded by a red 

 areola, and if a photograph be taken at this point, it 

 will be found perfect, as the violet rays are the most 

 chemically active. This is owing to the different 

 refrangibility of the red and violet rays, the latter 

 coming to a focus sooner than the former. When 

 photographing, the point V should be chosen where 

 the object is surrounded by a red areola, this being 

 the most accurate for the violet rays. 



Microscopic objectives have of late years arrived at 



