1899] MICROSCOPICAL JOURNAL. 301 



the new system that it is easy to use eyepieces magnifying 

 8 to 10 diameters, often 18, and in special cases 27, with 

 rare instances of even 40. The evolution of thought is so 

 peculiar that a question is often here asked : "If this 

 property for bearing high eyepieces be true, and certainly 

 it is, what is the use of making high power objectives at all 

 seeing that so much magnification can be done at the eye- 

 end with eyepieces ? To reply to this must be our next 

 subject. 



In the olden days or up to about the year 1874, power 

 pe?' se was all that was thought to be effective in making 

 an object distinct, and its details plainly visible. It is 

 /lot a little strange to think that up to that date there ap- 

 pears to have been an entire absence of knowledge, even 

 by experts, both in theory and practice as to the real op- 

 tical principles that enable one to see an image produced 

 by the microscopic objective. But in 1877 or thereabouts, 

 Prof Abbe gave to the world his great diiFraction theory 

 which has inaugurated an entirely new epoch in both the 

 theory and construction, as well as the adaptation and use 

 of microscopical lenses. But before this time, however, 

 as knowledge has grown and experience accumulated, it 

 had come to be very slowly recognized that a "something'' 

 beyond simple magnification affected the deting power of 

 objectives, and at last it was in a great measure traced to 

 the fact that the reason why one lens defined so much bet- 

 ter than another, although of the same initial magnify- 

 ing power, was due to the fact that one had a greater ap- 

 perture than the other. Hence to increase the aperture 

 rapidly became the aim of the optician. But here it should 

 be distinctly stated that the aperture, or numerical angle 

 of the objective must not be confounded with the still older 

 idea that excellency of result depended on the angle of 

 obliquity at which the light emerged from or to the object, 

 involving as it did some specially assumed property of a 

 special kind in the obliquity taken as such,f or Prof. Abbe 



