444 ON" APERTURE AND DEFINITION OF MICROSCOPE OBJECT GLASS. 



cone of light beneath the object. The angular divergence of the 

 pencils issuing from the object and incident upon the objective 

 being regulated by that of the illuminating cone, every one who 

 uses a diaphragm between the mirror of his instrument and the 

 object on its stage (or any arrangement employed for the same 

 purpose) practically recognises and acts upon the fact (whether 

 understood or not) that delineation of the microscope iiLage is best 

 regulated by comparative trial of the different sized openings in the 

 diaphragm. Or, in other words, by suiting the angular divergence 

 of the defining pencils — it is the light which defines - to the 

 capacity of the lens used. And a very little experience suffices to 

 show that the effect of a larger or smaller illuminating cone stands 

 in direct relation with the magnifying power used. With low and 

 moderate powers, the best ''absorption" or ''negative" image is 

 obtained by using the diaphragm to shut off light, ix, by reducing 

 the divergence of the defining pencils, which thus enter the objective 

 with less inclination to the axis and form sharp points instead of 

 *' dispersion circles." "With higher and highest powers the 

 divergence of defining pencils may be extended until nearly the 

 full aperture of the objective is occupied, but definition becomes more 

 and more critical. In the first half of Helmholtz's essay the 

 relation of illuminating cone to amplification and brightness of 

 image is fully demonstrated, and it is shown that no diffraction 

 effects occur until the optical aperture through which the image is 

 viewed becomes smaller than the aperture of the pupil, but that 

 they increase at an enormous rate as the magnifying power is 

 increased. 



In the next place Professor Helmholtz, in dealing with the 

 question of extreme limits of resolution, obtains from his theory of 



diffraction the formula e — — ^ — . He does not, however, apply 



2 sm a 



it as expressing any value of "defining" power of aperture, but 



simply as expressing a certain relation h&tween the wave length of 



the different colors of the spectrum, and the aperture of an 



objective by which the smallest interspace between finely drawn lines 



