ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 359 



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

 but in air only four (see Figs. 108 and 109); the fifth and sixth are 

 completely lost in air, as there is no angle of obliquity whose sine 

 is >1. A dry objective of an angular aperture closely approaching 

 180° will 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 oj)tical continuity with the front of the 

 lens. 



Thus again it is shown, in agreement with the dioptrical 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°. 



Eecalling what was before explained as to the progressive enlarge- 

 ment that takes place in the diameters of the emergent beams of dvj, 

 water-immersion, and oil-immersion objectives of maximum aperture 

 (see Fig. 54), it will be readily seen how the enlarged diameters will 

 allow of the admission of additional diftVacted beams. 



The action of the obliquity of the incident light may also be seen 

 by comparing Fig. 108 with Fig. 57. In the latter case the direct 

 beam is no longer vertical but horizontal, so that new spectra can 

 now enter the field, and particular structure before invisible comes 

 into view. 



These conclusions are simply inferences from two well-established ^ 

 optical laws — that of Fraunhofer, and the law of refraction. They 

 are supported to the full extent by observation under the Microscope. 

 Though we cannot see the contraction of the diffraction group in the 

 denser medium, and its subsequent extension by the transmission 

 into air, we can directly observe the various beams at their entrance 

 into the objective by observing the spectra in the clear opening of the 

 system. In this way it is shown — 



1. If any structure is observed successively by a dry objective 

 of large air-angle, and by an immersion objective of moderate balsam- 

 angle (much less than the air-angle of the former), we never lose 

 any spectral beam which is visible with the dry objective. 



2. If we examine the same object with the same dry objective at 

 first in air, and afterwards in balsam, we always see the same spectra. 



3. Observing a balsam-mounted object with a wide-angled im- 

 mersion glass (balsam-angle > 82°) we see neio spectra which, under 

 the same illumination, are never taken up by any dry objective nor 

 by the immersion objective when the object is in air. 



V. The Value of Wide-angled Immersion Objectives. 



What is the result of the preceding demonstrations ? 



First. It is shown that, contrary to the "angular" theory, a 

 wide-angled immersion objective has an aperture (in the true and 

 legitimate sense of the term as meaning " opening ") exceeding that 



