ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



623 



position, making the optical tube-length 252 instead of 270 mm., so 

 that for this reason it would magnify about 7 per cent, too httle. 



The result of the combination of both ' effects is, therefore, that the 

 Holos 8 mm., although really of 7 • 8 mm. focus, nevertheless magnifies 

 nearly 5 per cent, less than a true 8 mm. fulfilHng Abbe's assumption 

 as to position of focal plane. 



Determination of Numerical Aperture. — ThiQe Microscopes were 

 adapted to demonstrate the use of the Abbe apertometer and certain 

 dangers to be avoided. The latter refer to the probability of obtaining 

 too low a reading when measuring objectives or condensers with a large 



Fig. 101. 



clear aperture, owing to the auxiliary Microscope not having a sufficiently 

 large field to receive light from the marginal zone of objectives and 

 condensers of this type. Figs. 101 and 102 show clearly how this trouble 

 arises, and the causes to which it is due. 



In fig. 101 it is shown how the cone of rays proceeding from the large 

 objective is of too large a diameter to be completely received by 

 the auxiliary objective A A supplied with the apertometer, the result 



Fig. 102. 



being that only light corresponding to the dotted cone enters the 

 auxiliary Microscope, and that if a measurement is attempted under these 

 conditions, the numerical aperture of the dotted cone instead of the full 

 cone will be obtained. 



Another way in which the trouble arises is illustrated in fig. 102. In 

 this case the auxiliary objective A A is large enough to receive all light 

 coming through the objective 0, but owing to an excessively long 

 draw-tube, or to the fact that an eye-piece of too high a power is 

 used, the marginal part of the light is spread out too much to enter 

 the eye-piece, and is thus cut off at this end of the instrument ; and by 



