42 SELECTION AND USE 



which do not belong to the objects, and finally, if the outlines 

 of the object should happen to be straight lines, and be known 

 to be such, it will be found that they will appear to be curved 

 and distorted. It is evident, therefore, that a simple lens can- 

 not be used as an objective in any important work; its indica- 

 tions are unreliable, and the imagination is allowed full scope, 

 so that the eye is enabled to see whatever the mind desires 

 to see. 



The defects which we have just detailed, and which are found 

 in every simple glass lens, whose surfaces are bounded with 

 curves that are parts of circles, are largely due to what is called 

 spherical and chromatic aberration. As these terms are probably 

 not familiar to many of our readers, we will give as full and 

 simple an explanation of the subject as can be done without 

 the formal aid of mathematics. 



Spherical Aberration. The enlarged image formed at 

 the focus of any lens, and rendered visible on a screen or sheet, 

 is produced in this way: The rays proceeding from the object, 

 and passing through the lens, are, by the action of the glass, 

 bent from the path they would otherwise pursue. The object 

 may of course be supposed to consist of an infinite number of 

 points, and from these points rays proceed in every direc- 

 tion, and consequently through every part of the lens. If the 

 lens were perfect, all the rays from any one point would be 

 brought together at a second point corresponding with the 

 first. Unfortunately, however, the ordinary lens does not do 

 this; the central portions of the lens and the outer por- 

 tions act differently; the one brings the rays to a focus at 

 a point a little nearer to the lens than the other, and, con- 

 sequently, although we move the screen to a slightly greater 

 or less distance, we sfcill get an image of about the same 

 degree of distinctness. It is obvious, therefore, that when 

 placed at any distance within certain limits, the screen will 

 receive not one image, but a series of layers of images as it 

 were, and this consequently gives an indistinctness to the 

 resulting image. 



Our readers will find no difficulty in thinking out this 

 matter for themselves, and when they have arrived at clear 



