In practice, there are mainly two f

In practice, there are mainly two factors that considerablyintensify spherical aberration:1. The difference in the refractive indices of theimmersion medium and mounting medium. Themore the refractive index of the immersionmedium deviates from the refractive index ofthe mounting medium, the more marked thespherical aberration. An objective that, for example,is calculated for oil immersion (n = 1.52)therefore demonstrates a considerable sphericalaberration if the specimen structures are imagedin a watery solution (n = 1.33).2. The distance between the cover glass and thespecimen structure to be examined. In general,spherical aberration intensifies as the sampledepth increases. Ideally, therefore, the specimenshould be positioned directly under the coverglass.