84 THEORY OF THE MICROSCOPE. 



diameter, whose centre coincides with the corresponding image- 

 point of the concentric system. 



Similar phenomena are also observable, if, instead of the whole 

 objective, single defectively centred lenses are turned round their 

 axis. The displacements may, indeed, prove still more consider- 

 able, because aberrations, which in other cases had a counteracting 

 effect, have, in consequence of the rotation, to be added to the 

 others. Moreover, no such evident relation, as usually supposed, 

 exists between the inclination of the individual axis and the 

 resulting displacement of the objective-image. When, for instance, 

 Harting states that every irregularity is magnified exactly as 

 much as the object itself, and that therefore a difference of 10 mic. 

 in an image magnified 500 linear becomes 5 mm., it is an entirely 

 erroneous assertion, which finds a sufficient refutation in the 

 example itself. On the contrary, it is always conceivable that the 

 defects of centering may cancel each other, so that the resulting 

 image does not suffer any displacement, although it loses in dis- 

 tinctness. 



Moreover, in practice the ratios between the different 

 amounts of displacement are without significance, since the center- 

 ing of the objectives is always accomplished lens for lens, for 

 which purpose are usually employed not the dioptric images, 

 but the reflected images which the lens-surfaces form of a 

 luminous object placed on one side. [Cf. chapter on Testing the 

 Centering.] 



IX. 



BKIGHTNESS OF THE FIELD OF THE MICEOSCOPE. 



THE brightness of the field of a Microscope may be defined as 

 that amount of light which is brought to the unit of surface of the 

 retina, when a uniformly luminous surface of known intensity 

 serves as the object. The ratio of this quantity of light to that 

 which the same illuminating surface brings to a unit of surface 

 of the retina with the naked eye represents its arithmetical 

 expression. It is not a quantity dependent upon the distance 

 from the eye, for in the retinal image a change in the distance of 

 any object becomes larger or smaller in the proportion in which 



