ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 



133 



supposed, ttere is a maximum angular aperture which cannot be 

 surpassed without undergoing a perceptible loss of definition pro- 

 vided a certain working distance is required. 



Withdrawing now the front aberration by an immersion fluid 

 which is equal to crown glass in refractive power, and withdrawing it 

 for all colours at the same time by selecting a fluid similar to crown 

 glass in dispersive power likewise, will at once remove the difficulty. 

 Consider for example an apertui-e of 1*25 (numerical). Water being 

 prescribed as immersion fluid, the front aberration would affect a 

 pencil of 140°, containing rays up to an obliquity of 70°, and with 

 strong glycerine of 1'45 this latter angle would remain 63° still. 

 Substituting a medium which performs like fluid crown glass, the 

 same pencil (contracted to the equivalent angle of 112^) will be 

 admitted to the front lens without any aberration, and owing to the 

 performance of the Amici type of construction, may be made to 

 emerge from the curved surface of the front lens without any detri- 

 mental aberration, but contracted to an angular aperture of 70° to 90°. 

 The first notable spherical aberration of the pencil then occurs at the 

 anterior surface of the second lens, where the maximum obliquity of 

 the rays is considerably diminished already, 



A numerical aperture of say 1 • 25 represents a water angle = 140°, 

 a glycerine angle of 126°, and a crown-glass angle of 112°. If, now, 

 such an objective of 1*25 should be made for working with water 



Fig. 20. 



Fig. 21. 





(Fig. 20), there would be a cone of rays extending up to 70° on both 

 sides of the axis, and this large cone would he submitted to spherical 

 aberration at the front surface a ; with glycerine this would be similar, 

 though in less degree. 



But if there is homogeneous immersion (Fig. 21), the whole cone 

 of 112° angular aperture is admitted to the front lens without any 

 aberration, because there is no refraction at the plane surface. And as 

 the sphericcd surface of the front lens, though it may effect a consider- 

 able refraction, is without notable spherical aberration, the incident 

 pencil likewise is brought from the focus F to the conjugate focus F', 

 and contracted to an angle of divergence of 70-90° without having 

 undergone any spherical aberration at all. 



Thus the problem of correcting a very wide-angled objective is reduced 



