82 VISION WITH THE COMPOUND MICROSCOPE 



It will be ' empty amplification/ because there is nothing in the 

 image which requires so much power for distinct recognition. If 

 the power be deficient, aperture will not avail ; if the aperture be 

 wanting, nothing is gained by high power. If the angular aperture 

 of the microscope is such that the delineation of fine lines, whose 

 interspaces are one micron^ is just possible, it is fruitless labour to 

 increase the amplification beyond what we know to be sufficient for 

 their observation. We potentially differentiate what we are power- 

 less to see. 



Thus it may be inferred from the diffraction theory, as such, that 

 wide aperture should accompany high amplification, and moderate 

 aperture be the accompaniment of low or moderate amplification. 

 We have observed with great regret that students at our Biological 

 Schools in these days of low-priced objectives frequently abandon a 

 fairly good ^-inch objective of suitable numerical aperture, and 

 obtain in its place a | inch or T \ T inch with scarcely any increase of 

 numerical aperture, merely for the ease with which amplification is 

 effected. But it would be well to remember that high amplification 

 effects nothing unless accompanied by suitably widened aperture. 



The circumstances on which what has been called 'penetration ' 

 in objectives is dependent will be shortly considered ; 2 it may be 

 stated here that theory and experience alike show that ' penetration ' 

 is reduced with increasing aperture under one and the same ampli- 

 fication. As we have indicated, there are many subjects of study 

 and research presented to the biologist for which he needs as much 

 ' penetration ' as possible. This is always the case where the recog- 

 nition of solid forms as the infusoria, for example is important. 

 A fair vision of different planes at once is required. 3 Indeed the 

 greater part of all morphological work is of this kind ; here, then, 

 in the words of Abbe, ' a proper economy of aperture is of equal 

 importance with economy of pow T er.' 4 



Whenever the depth of the object or objects under observation 

 is not very restricted, and for the purposes of observation we require 

 depth dimension, IOW T and moderate powers must be used ; 'and no 

 greater aperture should therefore be used than is required for the 

 effectiveness of these pow-ers an excess in such a case is a real 

 damage.' 5 



Moreover, in biological work constant application of the instru- 

 ment to varied objects lenses of moderate aperture and suitable 

 power facilitate certainty of action and conserve labour. Increase 

 of aperture involves a diminished working distance in the objective, 

 and it is inseparable from a rapid increase of sensibility of the 

 objectives for slight deviations from the conditions of perfect cor- 

 rection. If it be not necessary to encounter the possible difficulties 

 these things involve, to do so is to lose valuable moments. These 

 difficulties, of course, are diminished by the use of homogeneous, and 



1 A micron is M^ToVo mm - Vide Journ. E.M.S. 1888, pp. 502 and 526; and 

 Nature, vol. xxxviii. pp. 221, 244. - See p. .So. 



" Abbe's explanation of the reason of the non-stereoscopic perception of these is. 

 given (see pp. 93 et seq.}. 



* ' The Relation of Aperture to Power,' Journ. P. M.S. series ii. vol. ii. i>. 304. 



* Ibid. 



