THE IMMERSION APERTURE. 77 



fluid abolished the refractive action of the first plane 

 surface which, in the case of air, prevented there being 

 any pencil exceeding 82 within the glass. Also the very 

 curious mistake which arose from the assumption that 

 a hemisphere did not magnify an object at its centre 

 because the rays passed through without refraction. 

 A further erroneous view has, however, been so wide- 

 spread that it will be desirable to devote a few lines 

 to it, especially as it always seems at first sight to be 

 both simple and conclusive. 



If a dry objective is used upon an object in air as 

 in fig. 42, the angle may approach 180, but when the 

 object is mounted in balsam as in fig. 420, the angle 

 at the object cannot exceed 82, all rays outside that 

 limit (shown by dotted lines) being reflected back at 

 the cover-glass and not emerging into air. On using 

 an immersion objective, however, the immersion fluid 

 which replaces the air 

 above the cover-glass 

 allows the rays former- 

 ly reflected back to pass 

 through to the objec- 

 tive so that the angle 

 at the object may again 



be nearly 180 as with the dry lens. The action of the 

 immersion objective was, therefore, supposed to be 

 simply that it repaired the loss in angle which was 

 occasioned when the object was transferred from air to 

 balsam, and merely restored the conditions existing in 

 fig. 430 with the dry objective on a dry object. 



As the result of this erroneous supposition, it fol- 

 lowed that an immersion objective could have no 

 advantage over a dry objective, except in the case of 

 the latter being used upon a balsam-mounted object, 

 its aperture then being (as was supposed) " cut down." 

 The error lies simply in overlooking the fact that the 

 rays which are reflected back when the object is mounted 

 in balsam (fig. 420) are not rays which are found when 

 the object is in air (fig. 42), but are additional and 

 different rays which do not exist in air, as they cannot 

 be emitted in a substance of so low a refractive index. 



