HOMOGENEOUS VERSUS DRY OBJECTIVES 79 



according to the law of refraction, this group, on passing to air by 

 the plane surface of the covering-glass, is spread out the sines of 

 the angles being compared in the ratio of the same refractive index. 

 Consequently the various diffraction pencils, the first, second, . . . 

 on every side, after their transmission into air, have exactly the 

 same obliquity which they have in the case of direct emission in 

 air from an uncovered object. 



If now any dry objective of, say, 133 air-angle is capable of 

 admitting a certain number of these pencils from the uncovered 

 object, it will admit exactly the came pencils from the balsam- 

 mounted object. The contracted cone in balsam of 75 angular 

 aperture embraces all rays which ark ^emitted in air within a cone of 

 133. 



The aperture of an objective is not, therefore, cut down by 

 mounting the object in a dense medium, for no ray which could be 

 taken in from the uncovered object is lost by the balsam-mounting. 



3. A comparison of figs. 66, 67, and 68 will show that a cone of 

 82 within the balsam medium embraces all the diffracted rays 

 which are emitted from the object in air or transmitted from balsam 

 to air. This, however, is not the totality of rays which are emitted 

 in the balsam. The formula of Fraunhofer shows that the number 

 of the emitted beams is greater in balsam than in air in the same 

 ratio as the refractive index. 



A structure the distance of whose elements equals 2'2^t emits in 

 balsam six distinct beams on each side of the direct beam, but in air 

 only four (see figs. 66, 67, and 68); the fifth and sixth are completely 

 lost in air. A dry objective of an angular aperture closely approaching 

 180 w T ill not even take in the fourth deflected beam, as this is de- 

 flected at an angle of 90. But any immersion-glass of a balsam- 

 angle slightly exceeding 82 will take in the fourth, and if the 

 balsam-angle should exceed 112 it will take in the fifth beam also, 

 provided the object is in balsam, and in optical continuity with the 

 front of the lens. 



Thus, again, it is seen (as was before shown by the purely dioptric 

 method) that an immersion objective of balsam-angle exceeding 82 

 has a wider aperture than any dry objective of maximum angle can 

 have, for it is capable of gathering in from objects in a dense medium 

 rays which are not accessible to an air-angle of 180. 



It is, then, by the above facts and reasoning, placed beyond all 

 dispute 



1 . That a wide-angled ' immersion ' or ' homogeneous ' objective 

 possesses an aperture in excess, of 180 ' angular aperture ' in air ; 



2. That the great value of this always manifest practically is 

 fully accounted for and explained by the diffraction theory of micro- 

 scopic vision ; and 



3. That * dry ' objectives, so far as regards the perfect delineation 

 of very minute structures, can only be considered as representing an 

 imperfect phase of construction. When made by the best hands, 

 with every precaution and care employed to secure the best possible 

 corrections, their defects do not lie in the direction of the presen- 

 tation of false or even partially erroneous and distorted images. 



