18 THE STRUCTURE OP 



be expected from it, upon a correct theory of the 

 principles upon which it is constructed. Neverthe- 

 less, this instrument did not come perfect from its 

 inventor's hands. Its principles were understood by 

 the earlier microscopic observers in the seventeenth 

 and eighteenth centuries, but there were certain 

 drawbacks to its use, which were not overcome till 

 the commencement of the second quarter of the 

 present century. 



These drawbacks depended on the nature of the 

 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 is produced. These colours, 

 whfln formed by a prism, produce a coloured image 

 called the spectrum. Now, all pieces of glass 

 with iiTegular 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 Microscope, the greater, of course, 

 is the liability to this colouring. This is chromatic 

 aberration ; and the liability to it in the earlier- 

 made Compound Microscopes 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 

 distorted. This is spherical aberration, and a fault 



