Ch. IX] PAR-FOCAL OCULARS AND OBJECTIVES 311 



On page 31 of the Bulletin is the following: " Par-focal Eye-pieces. 

 Referring to the article in the April issue of the Bulletin, on changing 

 eye-pieces without altering focus, etc., we announce that we are pre- 

 pared *to furnish eye-pieces as here described with our Acme micro- 

 scopes at a sHght additional expense. 



We have named these eye-pieces Par-focal, meaning of equal 

 focus, from the Latin par (equal) and focus." 



For the par-focaHzation of objectives, see § 74-75. 



COLLATEKAl READING FOR CHAPTER IX 



Optic Projection, S. H. & H. P. Gage. 



Principles of Microscopy, Sir. A. E. Wright. 



Microscopy, E. J. Spitta. 



The Microscope and its Revelations, Carpenter-Dallinger. 



Journal of the Royal Microscopical Society. 



Transactions of the American Microscopical Society, especially the address of 

 Hon. J. D. Cox, 1884, pp. 5-39 on Aperture, and 1893, pp. 1-16, and A. C. Mercer, 

 1896, pp. 321-396. 



C. W. Woodworth, A. New Fundamental Equation in Optics. Science, N. S. 

 Vol. XLni, pp. 824-825. 



John C. Shedd, The Index of Refraction. School Science and Mathematics, 

 Vol. VI., 1906, pp. 678-680. 



(This article gives a brief history of the discovery of the law of refraction; 

 it also discusses the ratio of velocities in different media, and shows that the 

 coefficient of retardation of velocity in a transparent medium is the reciprocal 

 of the index of refraction.) 



