VISION WITH THE COMPOUND MICROSCOPE 



occupy the whole hemisphere (fig. 40) are in oil compressed within 

 an angle of 82 round the direct beam (fig. 41), so that there is room 

 for additional beams.' 



The unequal equivalent of equal angles becomes, therefore, a de- 



FIG. 40. Diffracted beams in air. 



Fin. 41. Diffracted beams in oil. 



monstrated truth a truth which is capable of experimental proof by 

 every owner of a fair microscope. 



Any one possessing a dry object-glass of an aperture of 170, for 

 example, may readily do so. In this case, a, , fig. 42, will represent 



a a 



170 IN AIR 



the pencil radiating from an object in air, and capable of being 

 taken up by that objective. This pencil, on its emergence from the 

 back lens of the combination, will present a diameter somewhat less 

 than twice the focal length of the objective presented in fig. 43. 

 But let the object be now placed in Canada balsam and 

 covered in the usual way ; the angle of the pencil, by 

 the greater refractive power of the medium, will be de- 

 monstrably reduced to 80, as shown in fig. 44. But it 

 will lie found, on examination of the emergent pencil 

 from the back lens, that this pencil occupies exactly the 

 same diameter (fig. 43) as before. The medium in which 

 the object is 1ms not, of course, altered the power of the 

 objective ; and since the diameter of the emergent pencil is the same 

 in both eases, the ratio of ' opening' to focal length, which is the 

 aperture, is the same also. Hence it is seen in the simplest way 

 that different angles in media of different refractive indices may 



170" IN AIR 



FIG. 43. 



-, BALSAM 



FIG. 44. 



denote equal apertures, and t'<jnl angles in different media denote 

 different apertures. 



That 'immersion' objectives may have gre.-iter apertures than 

 the maximum attainable by a dry objective is capable of equally 

 proof by accessible experiment. 



If an oil-immersion objective of 122 balsam angle be taken, and 

 so illuminated that the whole aperture is filled with the incident rays, 

 and if we use first an object mounted in air, we really find that we 



