74 THE MICROSCOPE. 



transmit more rays from the object to the image than 

 any dry objective is capable of transmitting. When- 

 ever the angle of an immersion lens exceeds twice the 

 critical angle for the immersion fluid, i.e., 96 for water 

 or 82 for oil, its aperture is in excess of that of a dry 

 objective of 180. 



This excess will be seen if we take an oil-immersion 

 objective of, say 122 balsam-angle, illuminating it so 

 that the whole field is filled with the incident rays, and 

 use it first on an object not mounted in balsam, but dry. 

 We then have a dry objective of nearly 180 angular aper- 

 ture, for, as will be seen by reference to fig. 41, the 

 cover-glass is virtually the first surface of the objective, 

 as the front lens, the immersion fluid, and the cover- 

 glass are all approximately of the same index, and 



FRONTLENS 



____^__ IMMERSION FLUID 



DBJUCTINMR __r^jr^ni COVE* CLASS 

 SLIDE 



FIG. 41. 



form, therefore, a front lens of extra thickness. When 

 the object is close to the cover-glass the pencil radiating 

 from it will be very nearly 180, and the emergent 

 pencil (observed by removing the eye-piece) will be seen 

 to utilize as much of the back lens of the objective as 

 is equal to twice the focal length, that is the inner of 

 the two circles at the head of fig. 40. 



If now balsam is run in beneath the cover-glass so 

 that the angle of the pencil taken up by the objective 

 is no longer 180, but 122 only (that is, smaller), the 

 diameter of the emergent pencil is larger than it was 

 before, when the angle of the pencil was 180 in air, 

 and will be approximately represented by the outer 

 circle of fig. 40. . As the power remains the same in 

 both cases, the larger diameter denotes the greater 



