52 Modern Microscopy 



Spherical Aberration. — Bays of light passing through 

 the marginal portion of a lens come to a focus nearer to 

 the lens itself than those rays which pass through the 

 centre of the lens, and the interval between the focal points 

 of rays which pass through the marginal and the central 

 parts of that lens is the spherical aberration. In compound 

 lenses this spherical aberration can be corrected for one or 

 more special ra,ys, and a lens so corrected is called aplanatic. 

 It is only truly aplanatic for the particular rays for which 

 it has been accurately corrected. 



Spherical Over-correction is present when a lens limits the 

 marginal rays at a greater distance than the central rays. 

 Spherical over-correction is indicated when the marginal 

 rays focus closer to the lens than the central ones. 



Spherical Zones. — In objectives of considerable aperture 

 the intermediate rays may show decided spherical aberra- 

 tion, although the central and marginal rays are united. 

 This defect is meant when spherical zones are spoken of. 

 The degree to which spherical zones are corrected deter- 

 mines chiefly how large a cone of illumination, and how 

 deep an eyepiece an objective will bear before ' breaking 

 down.' A high degree of correction for spherical aberration 

 and spherical zone must accompany the reduction of 

 chromatic defects before terms such as ' semi-apochro- 

 matic,' and especially ' apochromatic,' can be applied to a 

 lens. 



Stop. — In an optical instrument this is a means of 

 obstructing the passage of the central portion of a beam of 

 light. 



MAGNIFYING POWER. 



It is often supposed by the novice that magnification is 

 •dependent on the size of the instrument, and a large one is 

 frequently described as a very powerful one. This is quite 

 an error. Given suitable eyepieces and objectives, the same 

 magnification may be obtained on a small microscope as on a 

 large one. It is entirely dependent on the two optical parts, 



