THE IMMERSION SYSTEM. 85 



system, both in theory and in practice, to the prob- 

 lem of correcting an objective having a moderate air 

 angle. 1 



The adoption of the Homogeneons-immersion system 

 is at present warmly advocated by all opticians. 

 Messrs. Powell and Lealand take the lead with their 

 new formula |, which, illuminated by their oil immer- 

 sion-illuminator, will resolve the most difficult test- 

 objects. This objective has an aperture, measured in 

 crown-glass, of 150,= 1*47 N.A. "the widest aperture 

 yet produced." The same firm have constructed a -^ 5 

 and a -fj on the homogeneous system, but the apertures 

 are not so high as the ^th. " By the homogeneous- 

 immersion formula adopted by Powell and Lealand 2 

 the focal distance is practically a constant quantity, 

 and it follows that reduction of aperture by making 

 the front lens thinner ensures a much greater working 

 distance without affecting the aberrations ; for as the 

 first refraction takes place at the posterior or curved 

 surface of the front lens, the removal of any portion of 

 thickness at the anterior or plane surface simply cuts 

 off zones of peripheral rays without altering the dis- 

 tance any space being filled up by the homogeneous- 

 immersion fluid, or by an extra thickness of cover- 

 glass. 



" By applying an extra front lens to the back construc- 

 tion of such a -j^ th, the observer is enabled to view an 

 object through a cover-glass that would be practically 

 a maximum thickness for a -Jth (aperture =90) con- 

 structed on the usual formula where the setting en- 

 croaches on the active spherical refracting surfaces. A 

 second front might give a high average aperture for a 

 -j^- (115), while the thickest front representing the 

 maximum aperture of the whole construction (142) 

 would enable the observer to view an object with a 

 greater aperture than has hitherto been obtained with 

 any T V* n > owing to difficulties of construction, and 

 through a thicker cover-glass than a y^th of the same 



(1) See Prof. Abbe " On Stephenson's System of Homogeneous Immersion 

 for Microscope Objectives," Journ. R. Micr. Soc., II. (1S79), p. 256, and on 

 'The Essence of Homogeneous Immersion ." Ibid., I. (1881), p. 13L, 



(2) Jovial of the R. M. S., Vol. III., p. 1050 (1880;. 



