364 OBJECTIVES, EYE-PIECES, THE APERTOMETEK 



construction of objectives and eye-pieces, and, as a consequence, has 

 to some considerable extent given a new character to the entire in- 

 strument. Its promulgation has indeed inaugurated an entirely 

 new epoch in the construction and use of the microscope. 



The general character and the details of Abbe's theory are given 

 in the second chapter of this treatise ; but its practical bearing upon 

 the theory and application of the optical part of the instrument was 

 soon manifest ; for in 1878 the homogeneous system of immersion 

 objectives l was introduced as a logical outcome of the diffraction 

 theory of microscopic vision. A formula for a ^-inch objective on 

 this system was prepared by Abbe, to whom, we learn from himself, 

 it had been suggested by Mr. J. "W. Stephenson, of the Royal 

 Microscopical Society. 2 It has been already shown 3 that the homo- 

 geneous system was so called because it employed the oil of cedar- 

 wood to unite the front lens of the objective to the cover-glass of 

 the object, in the same way as water had been employed in the 

 ordinary immersion system, ; but as there was a practical identity 

 between the refractive and dispersive indices of the oil and those 

 of the crown glass of the front lens, the rays of light passed 

 through what was essentially a homogeneous substance in their 

 path across from the balsam-mounted object to the front lens, and 

 a homogeneous system of objectives took the place of the previous 

 water immersions. 



This was the first great step in advance in optical construction 

 and application following the theory of Abbe. 



As often happens in matters of this kind, there had been an 

 apparent anticipation of this system of lenses by Amici as far back 

 as 1844 ; but it is very apparent that Amici employed the oil of 

 aniseed without any clear knowledge of the principles involved in 

 the homogeneous system, being wholly unaware of either the increase 

 of aperture involved or the cause of it. But this cannot be said of 

 Tolles, of New York. We have pointed out that, as early as 1873, 

 he made a j^j-inch, and subsequently, in the same year, a |-inch 

 objective, each with a duplex front to work in soft balsam, and with 

 a N.A. of 1'27. These objectives were examined by the late Dr. 

 Woodward, of the Army Medical Department, New York, and with 

 that examination were allowed to drop. For Tolles as an original 

 deviser of a practical homogeneous system this was unfortunate ; for 

 the actual introduction of the system in a form capable of universal 

 application, and worked out in all its details in an entirely inde- 

 pendent manner, we are wholly indebted to Abbe. 



The principle was not, nevertheless, so readily and warmly 

 adopted in England on its first introduction as might have been 

 anticipated. This arose partly, however, from the fact that water 

 immersions had been brought to so high a point of excellence by 

 Messrs. Powell and Lealand that the early homogeneous objectives 

 were not possessed of more aperture, and were not sensibly 

 superior to the best immersions made in England. 



The homogeneous objectives were made with duplex fronts and 



1 Chapter II. 2 P. 27 ; also Journ. Boy Micro. Soc. Vol. II. 1879, p. 257. 

 5 Chapter I. 



