SPHERICAL DISTORTION. 



focus, be viewed by an eye at E, an imaginary image will be seen 

 at a certain distance, greater than that of the object. 



Fig. 29. 



If the object be straight or flat, such as o' o', the image will be 

 curved with its convexity turned towards the lens, as shown at 

 i' i', in the figure. If the object be concave towards the lens, the 

 image will be less and less convex, until the object having a 

 certain concavity, such as o" o", the image will be straight or 

 flat as shown at i" i". If the concavity towards the lens be still 

 greater, as at o'" o'", the image will become concave towards the 

 lens, but less so than the object. If the object be convex 

 towards the lens, as at o o, the image 1 1 will also be convex 

 towards it. 



It follows, therefore, that a straight or flat object seen through a 

 convex lens thus will appear curved or convex, and that a convex 

 object will appear more convex. A concave object, provided it 

 have a certain degree of curvature, will have a straight or flat 

 image, and all objects more concave will have concave images. 



These results will be found to have considerable importance in 

 the practical construction of compound microscopes. 



48. From what has been explained it follows, that if any 

 expedient could be discovered, by which the focal length of a lens 

 could be shortened without increasing its convexity, we could 

 obtain a given magnifying power with a lens of a given 

 diameter without increasing the aberration, a result which would 

 be a most evident advantage. Now, there is only one way by which 

 this could be accomplished, which is by finding some material for 

 the lens, which without any countervailing disadvantages would 

 have a greater refracting power than glass. A lens made of such 

 a material would have a shorter focus, and consequently a greater 

 magnifying power than a lens of glass with the same convexity. 



107 



