Olf THE THEORY OP THE MICROSCOPE. 217 



resolves itself into a series of independent elements which as they 

 increase in number, follow, with the increasing inclination of the 

 rays towards the axis, a more and more unequal course. An 

 absolute efFacement is only possible theoretically for the two first 

 members of the series. As soon as the angular aperture exceeds 

 a small number of degrees, the counteraction of spherical aberra- 

 tion can be effected in no other manner than by compensating the 

 irremoveable errors of the higher elements through intentionally 

 introduced residual aberrations of the lower ones. The accumula- 

 tion of unavoidable deficits which this method of compensation 

 necessarily leaves unremedied in consequence of the want of 

 uniformity in the course of the several elements, [compels a 

 limitation of the angle of aperture, otherwise this deficit will 

 afi'ect the microscopic image injuriously. Por angles of aperture 

 exceeding 60 » and a fortiori for the very large angles of modern 

 objectives, the theoretically indispensable pre-supposition of an 

 adequate compensation is found in the well-known type of 

 construction where a plain nearly hemispherical front lens is 

 combined with a strongly over corrected system of lenses. The 

 discovery of this mode of construction must be looked upon as 

 the basis of every improvement which has been introduced since. 

 Por a sfstem of lenses made to use in air, the limit of serviceable 

 aperture proves to be from 105^ to 110'', beyond which it is not 

 possible to counteract sufficiently the spherical aberration, except 

 by lessening the focal distance of front lens from the object to a 

 degree which makes it practically useless. The application of the 

 immersion principle renders it possible on the contrary to 

 overcome spherical aberration, where even the maximum angular 

 aperture is used, by proper adaptation of the altered relations 

 thus introduced. It is in this power of using very large angles 

 of aperture without proportional detriment to the correction of 

 spherical aberration, and also in avoiding the loss of light caused 

 in the dry system by reflection from the surface of the front lens, 

 that the real advantage of the immersion plan lies. It will 

 indeed be seen from what follows, that these two facts fully 

 explain the undoubted superiority of the immersion lens. 



