424 LIMITS OP OPTICAL CAPACITY OF THE MICROSCOPE. 



(or not greater) than the '-'■ normaV amplification, {i.e.j when the 

 area of the ocular image just fills the pupil) otherwise, with the same 

 condant divergence of incident rays, the brightness is inversely propor- 

 tional to the amplification of image. 



The normal amplification increases with the increase of the sine 

 of the divergence-angle whose greatest value is 1 when this angle 

 approaches a right angle, (as is the case with the widest-angled 

 ohjectives). 



Assuming 10 inches as the distance of clear vision for calculation 

 of the amplified image, and 1| mm. as radius of pupil for bright 

 illumination, the normal amplification is represented by the figures 

 166.7, and the brightness of image follows the following rates : 



Tor an amplification of 333.3 ^brightness. 



„ 500.0 -I 



„ 666.7 3-V „ 



"Which shows how rapidly the bright ness must necessarily decrease 

 with increasing amplifications. 



"Were it possible to conduct a hemispherical cone of light from 

 an object lying in water into an immersion lens, and form therewith 

 a correct image, all these amplifications might be raised in the pro- 

 portion 1.335 to 1 whilst the brightness of image remained the 

 same. But, as already remarked, every instrument hitherto con- 

 structed admits in air only, and not in water, a cone of incident light 

 at all approaching to the hemispherical (180**). 



The sectional area of the pencil of light entering the pupil may 

 be determined empirically with ease. Focus the instrument on a 

 bright field, and withdraw the eye from the ocular (keeping the 

 direction of the axis of the microscope) and look at the ocular 

 itself. Just in front of it will be seen a small bright circle against 

 a dark ground. This is the optical image of the objective lens 

 which the ocalar {i. e. chiefly its field glass) forms. All light which 

 comes through the objective and has passed the ocular must be 

 collected in this image of the objective. It corresponds, therefore, 

 to the area in which the several cones of light, transmitted from the 

 bright points of the object, are collected at this spot. To gather 



