THE VALUE OF WIDE APERTURES. 12$ 



aided, the human eye will only admit pencils of about 10, 

 and as light is dispersed from every point of an object to 

 an angle of 180, those glasses which approach infinitely 

 near this latter angle must give a more correct image of the 

 object that those of small apertures. 



When we appreciate the above facts we shall be able to 

 estimate the value of immersion objectives, which enable us 

 to collect and gather to a focus rays which cannot possibly 

 enter the microscope when a film of air exists between the 

 objective and object. The editors of the 'Journal of the 

 Royal Microscopical Society ' have given diagrams of the 

 relative diameters of the utilised back lenses of dry and 

 immersion objectives of the same power, commencing with 

 an air angle of 60 and ending with the homogeneous 

 hemisphere of 1 80 balsam angle, which has nearly the same 

 refractive index as crown glass. They also add, " Thus, if 

 we commence with an air angle of 10 and proceed by 

 successive additions of 10 up to 180 air angle, passing 

 then to 82 balsam angle, and again progressing to the 

 nearest practicable approximation to 1 80 balsam angle, 

 the emergent pencils will show a continuous increase ; there 

 is no break at 180 air angle, nor does anything abnormal 

 appear at that point, but we have a regularly progressive 

 series from the lowest air angle to the highest balsam angle." 



It has been urged by some, that all those rays equivalent 

 to more than 180 air angle are not image-forming rays, 

 but this assertion has also been dealt with in the same 

 journal. "The simplest experiment of all is to take a 

 homogeneous-immersion objective of large aperture, say 

 i "25 (i 10 balsam angle), and place a stop of tin-foil on the 

 back lens, leaving only a small clear annulus of the extreme 

 marginal rays. With sufficient obliquity of the illumination 

 the image of the object will be seen perfectly delineated 

 either on a bright or dark field." 



