THE MICROSCOPE. 33 



larging the field of view. It is called the field-lens. An 

 inspection of the dotted lines in the figure will show that many 

 of the rays pass beyond the reach of the eye-glass, B : an 

 image from these rays is represented at E. These rays are 

 intercepted by the field-glass D, and form an image at F, which 

 is viewed by the eye-glass. 



In looking through a common microscope of this kind, the 

 observer will probably see rings of color round the edge of 

 the field of view, and also similar colors around the edges of 

 the object he is viewing. These defects arise from the decom- 

 position of common white light j and are called chromatic 

 aberration or dispersion. The colors round the field of view 

 are produced by the defects of the eye-piece ; and those round 

 the object, by the object glass. Again : if the object be 

 looked at through the instrument as before, its outline or 

 edges will be observed, not sharp and distinct, but thick and 

 confused. This is caused by the rays not being collected into 

 a perfect point as they were on the object itself. This defect 

 is called spherical aberration. "When an instrument has 

 neither its chromatic nor spherical aberration removed, it is 

 said to be uncorrected. 



To conceal these defects there is generally a small hole or 

 stop behind the object glass. This is injurious to correct 

 vision, as it prevents a large portion of light from entering the 

 eye, and makes the objects appear dark, so that their true 

 structure cannot be made out. When this is the case, the 

 instrument is said to want angular aperture. The stop refer- 

 red to, however, is essential even to the moderate performance 

 of a common instrument. 



To obviate all these difficulties, improvements have been made 

 both in the object-glasses and the eye-pieces. Wollaston's 

 Doublet has been found capable, when used as an object-glass 

 with the Huygenian eye-piece (hereafter described), of trans- 



