66 Modern Microscopy 



glass surface, and positive in the front lens piano, the latter 

 preponderating owing to the greater diameter of the cone 

 ■of rays from the object where it enters the front lens. 

 With the correct thickness of cover-glass, the remaining 

 under-correction is exactly balanced by an equal over-cor- 

 rection in the lenses of the objective, but a thin cover-glass 

 produces insufficient over- correction, the diameter of the 

 cone of rays being too small when it meets the surface of 

 the cover-glass. A thick cover-glass produces the opposite 

 effect — that is, the cone of rays is too large. 



Low powers are not so sensitive to this influence as high 

 ones. There are two means of correcting this. Dry objec- 



FlO. 18.— COBEECTION COLLAK (ZeISs). 



tives having a large numerical aperture are often pro- 

 vided with what is termed ' a correction collar,' whereby 

 the two back combinations of the objective are removed 

 farther from, or brought closer to, the front lens or lenses. 

 Eig. 18 shows the manner in which this is effected in one of 

 Zeiss's lenses, bb is the correction ring, by turning which 

 the distance between the upper lenses and the two lower 

 lenses is varied. With such a correction collar a worker is 

 undisturbed by thickness of cover-glass, because he has 

 within certain limits the means at his disposal in the 

 objective itself for correcting same. The use of this cor- 

 rection collar almost requires a personal demonstration, 

 and to set it at the exact point that yields the best result 



