OF THE ificEoscbPE. 43 



ideas upon the subject, they will see that spherical aber- 

 ration is caused by the difference between the extent of the 

 refraction produced at different parts of the lens, and this 

 applies not only to all the rays proceeding from each in- 

 dividual point, but to the several pencils which proceed from 

 different points. 



It is evident that if some parts of the lens bring the rays to a 

 focus at a shorter distance than others, these parts must magnify 

 more, and such is in reality found to be the case. But if one 

 part of an object is magnified more than another, the image 

 will be distorted, and hence we have what is sometimes known as 

 aberration of form. This distortion is easily seen by examining a 

 piece of muslin with a magnifier of high power and large 

 diameter. The threads in the centre of the field of view* will 

 appear to be straight, while those at the outside will appear to 

 be curved.f 



Chromatic Aberration. This is a defect of ordinary 

 or uncorrected lenses, whereby they not only act as magnifiers, 

 but as prisms, decomposing the light, and causing objects seen 

 through them to appear with a fringe of color. Common hand 

 magnifying glasses, used in the ordinary way, do not exhibit 

 this defect to a very marked degree, but when the images formed 

 by lenses of this kind are again magnified, as is done in the 

 compound microscope or telescope, the color becomes very dis- 

 agreeably perceptible. 



* By field of view is meant that portion of the object which is visible 

 through the magnifier. 



fin ordinary lenses and microscopes, in which this defect is not cor- 

 rected by the structure of the glasses themselves, the effects of spherical 

 aberration are lessened by contracting the field of view, so that only those 

 parts of the object which are seen through the centre of the lens or objec- 

 tive are looked at. This contraction is usually effected by means of dia- 

 phragms, or round plates of metal pierced with a central hole, which are 

 so placed as to cut off the rays which pass through the edge of the lens, 

 and leave only those that are central. This plan, however, is only the 

 substitution of one defect for another, for by lessening the field of view of 

 the lens, we are prevented from seeing more than a very small portion of 

 the object, and in addition to this the light is so much reduced that tho 

 object is seen only with very great difficulty, and not at all clearly. 



