title: The Eye and Visual Optical Instruments
author:  George Smith 
description: 'The Eye and Visual Optical Instruments will not become dated quickly and covers a tremendous range of topics ... this book is an outstanding reference work. It is lucid, gives a good feel for important concepts to anyone willing to work through it, and is a gold mine of practical information. It is a must for any appropriate library and will have great longevity ... I recommend it to anyone with an interest in practical optics.' Nigel Douglas, Optics and Photonics News<br /><br />'Such a comprehensive book should certainly be bought by all optometry and vision science departments and it would not be out of place in psychology libraries. At a price of 35 (for the paperback version) it is just about in the price range of students as well.' Mark Scase, Bulletin of the Applied Vision Association<br /><br />'What I am looking for in an optics text is a book that simply explains the principle, builds on the basics and provides example, and problems for the student to practice ... The Eye and Visual Optical Insturments fills nearly all these roles admirably ... The language is beautifully clear, economical and complete in expression, and the diagrams are marvelously simple ...'. Andrew Carkeet, New Zealand Optics
zlib_text: [From the _Cambridge Philosophical Society Proceedings_ , Vol. I. pp.
  173–175.]
  
  
  X. _On the Elementary Theory of Optical Instruments._
  
  
  THE object of this communication was to shew how the magnitude and position of
  the image of any object seen through an optical instrument could be
  ascertained without knowing the construction of the instrument, by means of
  data derived from two experiments on the instrument. Optical questions are
  generally treated of with respect to the pencils of rays which pass through
  the instrument. A pencil is a collection of rays which have passed through one
  point, and may again do so, by some optical contrivance. Now if we suppose all
  the points of a plane luminous, each will give out a pencil of rays, and that
  collection of pencils which passes through the instrument may be treated as a
  _beam_ of light. In a pencil only one ray passes through any point of space,
  unless that point be the focus. In a beam an infinite number of rays,
  corresponding each to some 