unbalance small to medium-sized telescopes. Another disadvantage is a high purchase cost, with large Naglers' prices comparable to the cost of a small telescope. Hence these eyepieces are regarded by many amateur astronomers as a luxury. See also Barlow lens List of telescope parts and construction Optical microscope Monocle Optical telescope Pocket comparator References A. E. Conrady, Applied Optics and Optical Design, Volume I. Oxford 1929. R. Kingslake, Lens Design Fundamentals. Academic Press 1978. H. Rutten and M. van Venrooij, Telescope Optics. Willmann-Bell 1988, 1989. . P. S. Harrington, Star Ware: An Amateur Astronomer's Guide to Choosing, Buying, and Using Telescopes and Accessories: Fourth Edition. John Wiley & Sons, Inc. External links EYEPIECE EVOLUTION A. Nagler - United States Patent US4286844 A. Nagler - United States Patent US4747675 A. Nagler - United States Patent US4525035 A. Nagler - Finder scope for use with astronomical telescopes The evolution of the astronomical eyepiece, in-depth discussion of various design and theoretical background John Savard's Eyepiece Page, a list of eyepieces with some details of their construction. United States Patent Office: Ultra wide ocular NAGLER. Lenses Microscope components Telescopes