System of Homogeneous Immersion. By Prof. E. Abbe. 263 



connection of the front lens with the under surface of the shde by 

 a drop of water, is taken into account.* In the absence however 

 of an ilhiminating apparatus such as this, and where only very 

 oblique ilium inatiou is required, a much more simple arrangement 

 will be found very serviceable, which consists in connecting, by 

 means of a drop of glycerine or oil, a plano-convex lens, nearly 

 hemispherical, of 6-9 mm. radius, to the under surface of the slide, 

 to which it will adhere. It may be kept sufficiently centered by 

 means of a loose brass ring attached to it, having an external 

 diameter equal to that of the stage aperture. The ordinary 

 concave mirror, turned slightly outside the axis of the Microscope, 

 will then give cones of rays of any degree of obliquity which may 

 be desired. 



In conclusion, some account may be given of the optical com- 

 binations of the objectives for homogeneous immersion. Those 

 constructed in Mr. Zeiss' manufactory, and based upon my com- 

 putations, are all systems with four members. In this 1 have 

 gone back to a type of construction which was applied by me 

 experimentally many years ago, and has lately been used with 

 considerable success by several opticians, especially Mr. Tolles and 

 Mr. Spencer. Two single crown-glass lenses close together are 

 made use of (duplex front) as the lower members of the system, 

 and the two others only are compound, so-called achromatic (in 

 the present case binary) lenses. 



This form has certainly the disadvantage of leaving rather more 

 chromatic difference of the magnifying power (that is, with perfect 

 achromatism in the middle of the field of view there is more colour 

 towards the periphery) than is usually found when the front lens of 

 the system is followed immediately by a compound lens of flint and 

 crown glass ; but this defect is practically inconsiderable in com- 

 parison with the facility with which it enables the angle of aperture 

 to be increased. The form in which 1 have devised this type is 

 nevertheless essentially diflerent from the construction of which 

 Mr. Tolles has published the elements in detail. f The difference 

 becomes very ap})arent when the radii of the front lenses are compared 

 with the equivalent focal distances of the respective objectives. 

 The \" objective of Tolles, described in the journal referred to, hag 

 almost exactly 4 mm. focal length, and its front lens a radius of 

 • 73 mm. In Zeiss's ^Vj with 1 • 8 mm. of focal length — con- 

 sequently less than half — the radius of the front lens is no less than 



* In consequence of the greater aperture of the objectives for homogeneous 

 immersion, I have recently had a system of lenses constructed for an illuminatino- 

 apparatus, the angular nperture of which reaches approximately the numerical 

 equivalent 1*4. This will consequently give rays which are inclined 72° towards 

 the axis in glass. 



t This Journal, i. (1S78) 143 



