684 SUMMARY OF CURRENT RESEARCHES RELATING TO 



where again the amplification is referred to the conventional imago- 

 distance of 250 mm. 



By limiting more the amount of allowable indistinctness, all these 

 figures would be correspondingly reduced, and by enlarging the limit 

 they would be increased. On the other hand, they rise in proportion 

 to the refractive index if the objects are in water, balsam, &c. 

 In the same way they would be increased if ajierture-angles of less 

 numerical equivalent than 0*50 were made use of, as would always be 

 the case with low powers, and with higher powers if the illumination 

 was by narrow cones of rays, and the objects produced no perceptible 

 dissipation of the incident rays. 



It i-'s obvious that the actual deiJfh of vision must always be the 

 exact ' uni of the accommodation depth nud focal depth. The former 

 denotes that object space which the eye, through the play of accom- 

 modation, is able to penetrate with perfect sharpness of image ; the 

 latter gives the amount by which this object space is extended in its 

 limits — reckoning both from above and below — because without perfect 

 sharpness of image there still remains a sufficient distinctness of 

 vision. 



The very unequal course of the two constituent parts of the visual 

 depth appears directly from the two series of numbers given above, 

 but will be still more evident if we compare the depth values of both 

 series, calculated for the j^articular amplifications, with the lateral 

 diameter of the field for the same amplifications. The latter — the 

 linear field of vision of the Microscoj)e — depends exclusively upon the 

 amplification and upon the angle of field * of the eye-piece employed ; 

 and when the latter is taken as constant, is inversely proportional to 

 the amplification, whatever the construction of the Microscope may 

 be. If, for example, the diajjliragm of the eye-piece appears under a 

 visual angle of 53° (corresponding to the trigonometrical tangent 

 • 50 for the semi-angle), then the absolute diameter of the visible 

 field will be 



25 mm. for 10 times amplification. 



The accommodation depth (the same assumptions being made as in 

 the above example) will not remain a constant fraction of the field 

 of vision as would be the case if there were no over-amplification in 

 depth,! but would amount to — 



* The " angle of field " of an eye-piece is that angle under which the rays 

 emerge ■which meet opposite points of the diaphragm. It is at the same time the 

 visual angle under which the field is seen projected by the eye. 



t If the amplification of the object were increased 10 times, the absolute 

 diameter of the field would l>c reducf^d to ^l, and a similar reduction in the 

 deptli would leave the ratio of depth to field unchanged. 



