Page:Principles of Microscope.djvu/132

94 PRINCIPLES OF MICROSCOPY small obstacles in the form of dust upon the eye lens and eyelashes and intra-ocular opacities eclipse more and more of the image. Finally the retinal screen is so encumbered with shadows that distinct vision is at an end.

In point of fact, it is in the last instance, as the reader will see on referring to Cap. XVI and Plate XVIII, the narrowing of the beam and the projection of shadows upon the retina which imposes a limit upon the magnification used in the microscope.