Page:Principles of Microscope.djvu/140

100 For the attainment of this object it has been found necessary to over-compensate the objective and to under-compensate the ocular. The under-corrected oculars which complete the chromatic corrections of the apochromatic objectives are technically known as compensation-oculars. In the case of such under-compensated oculars the aperture of the combination which is brought into view by looking into the dismounted ocular in the direction of the window is fringed by an orange-coloured in lieu of, as in the ordinary ocular, by a blue band.