6 THE STRUCTURE OF 



lenses used in its construction. The technical term 

 for the defects alluded to, are chromatic and spheri- 

 cal aberration. Most persons are acquainted with 

 the fact, that when light passes through irregular 

 pieces of cut glass as the drops of a chandelier 

 a variety of colours are produced. These colours, 

 when formed by a prism, produce a coloured image 

 called the spectrum. Now, all pieces of glass 

 with irregular surfaces produce, more or less, the 

 colours of the spectrum when light passes through 

 them ; and this is the case with the lenses which are 

 used as object-glasses for Microscopes. In glasses of 

 defective construction, every object looked at through 

 them is coloured by the agency of this property. 

 The greater the number of lenses used in a Micro- 

 scope, the greater, of course, is the liability to this 

 colouring. This is chromatic aberration ; and the 

 liability to it in the earlier-made Compound Micro- 

 scopes was so great that it destroyed the value of 

 the instrument for purposes of observation. 



Again, the rays of light, when passing through 

 convex lenses, do not fall when they form a 

 picture all on the same plane ; and therefore, 

 instead of forming the object as presented, pro- 

 duce a picture of it that is bent and more or less 

 deformed. This is spherical aberration, and a fault 

 which was liable to be increased by the number of 

 glasses, in the same way as chromatic aberration. 

 This also occurred in Compound Microscopes; and 

 the two things operated so greatly to the prejudice 

 of this instrument, that it was seldom or never 

 used. 



Gradually, however, means of improvement were 

 discovered. These defects were rectified in tele- 

 scopes ; and at last a solution of all the difficulties 

 that beset the path of the Microscope-maker was 

 afforded by the discoveries of Mr. Joseph Jackson 



