THE INFLUENCE OF THE DIFFRACTION THEORY 363 



was entirely unknown. In the absence of this knowledge wholly 

 mistaken value was attached to power per se in the objective. 



With a focus as short as the ^--inch, it was not uncommon to 

 find apertures less than 1'2, while objectives of J*, $, -^ and even 

 higher powers, were made with extremely reduced apertures. This 

 was done in the interests of the common belief that ' power '- 

 devoid of its suitable concurrent aperture could do what was so 

 keenly wanted. 



Tins impression, however, was far from universally relied on ; 

 there were several earnest workers who, without being able to 

 explain, as Abbe subsequently did. why it was so, still urged the 

 opticians, in the manufacture of every new power, especially the 

 higher ones, to produce the largest possible amount of aperture; 

 and the evidence of this is still to be found in the objectives they 

 then succeeded in obtaining. But there can be no doubt that a 

 reckless desire for magnifying power, all other considerations apart, 

 greatly obtained ; and the opticians were able to encourage it, for it 

 is far easier to construct an objective of high power and low aperture 

 than it is to make a low power with a large aperture. 



FIG. 317. A suggested 

 combination oy Wen- 

 ham, 1869. 



FIG. 318. Combina- 

 tion for ' homoge- 

 neous ' immersion 

 by Abbe. 



FIG. 819. Diagram of 

 apochromatic com- 

 bination. 



Thus a, ^-inch of 0'65 N.A. will be far more expensive, and pro- 

 bably not as well corrected, as -,\ of O7 N.A. The i-inch objective, 

 even if a good one, is sure to exhibit spherical aberration, while the 

 jt- of low aperture will show many minute objects with considerable 

 clearness, especially if a comparatively narrow illuminating cone be 

 used. 



This difference becomes still more conspicuous as the difference 

 between aperture and power grows relatively greater, until we obtain 

 ultimately an amplification more than useless from its utter inability, 

 on account of deficiency of aperture, to grasp details. 1 



Up to 1874, however, there was an entire absence of knowledge, 

 even on the part of the leaders in microscopic theory, art, and 

 practice, as to the real optical principles that enabled us to see a 

 microscopic image, and consequently to understand the essential 

 requirements to be aimed at in the best form of microscope. But in 

 1877 Abbe's great Diffraction Theory of Microscopic Vision appeared, 

 which has led to changes of incomparable value in the principles of 



1 Vide Chapter II. 



