PENETRATING POWER IN OBJECTIVES 83 



especially apochromatic objectives, but even with these they are 

 not, in practice, eliminated where the best results are sought. 



Employ the full aperture suitable to the power used. This is the 

 practical maxim taught in effect by the Abbe theory of microscopic 

 vision. 



It has been suggested that all objectives be made of relatively 

 wide apertures, and that they be * stopped down ' by diaphragms 

 when the work of ' lower apertures ' has to be done. But this is 

 not a suggestion that commends itself to the working biologist. If 

 there were no other defects in such a method, the fact that the 

 working distance remains unaltered would be fatal ; and we may 

 safely adopt the statement of Abbe, 1 <that ' scientific work with the 

 microscope will always require, not only irigh power objectives of 

 the widest attainable apertures, but also carefully finished lower 

 powers of small and very moderate apertures. 



We complete this section with a table of numerical apertures, 

 which will be found on the following page. As already stated, the 

 resolving powers are exactly proportional to the numerical apertures, 

 and the expressions for this latter will allow the resolving power of 

 different objectives to be compared, not only if the medium be the 

 same in each, but also if it be different. The resolving power for an 

 objective, when illuminated by a ^ solid axial cone of white light, is 

 found by multiplying its X.A. by 70,000, and for monochromatic 

 blue-green light (Gifford's screen) by 80,000. 2 



The first column gives the numerical apertures from "40 to 1*52. 

 The second, third, and fourth, the air-, water-, and oil- (or balsam-) 

 angles of aperture, corresponding to every '02 of N.A. from 47 air- 

 angle to 180 balsam-angle. The theoretical resolving power in 

 lines to the inch is shown in the sixth column ; the line E of the 

 spectrum about the middle of the green (X = 0'5269/u) being taken. 



The column giving ' illuminating power,' we have already seen, 

 is of less importance ; while it must be borne in mind in using 

 the column of * penetrating power ' that several data besides. 



- go to make up the total depth of vision with the microscope. 

 a 



Penetrating Power in Objectives, Intelligibility and sequence, 

 more than custom, suggest the consideration of this subject at this 

 point. The true meaning and real value of * depth of focus,' or what 

 is known as ' penetrating power/ follows logically upon the above 

 considerations. 



That quality in an objective which was supposed to endow it 

 with a capacity of visual range in a vertical direction, that is, in the 

 direction of the axis of vision, has been called * penetration,' it 

 being supposed that by this ' property ' parts of the object not in 

 the focal plane could be specially presented, so as to enable their 

 perspective and other relations with what lies precisely in the focal 

 plane to be clearly traced out. 



Concerning the manner in which this quality of the objective 

 operated, there have been most diverse opinions ; indeed, the whole 



1 ' The Kelatioii of Aperture to Power,' Journ. B.M.S. series ii. vol. ii. p. 309. 

 3 Journ. R.M.S. (1893), p. 17. 



G 2 



