414 LIMITS OF OPTICAL CAPACITY OF THE MICEOSCOPE. 



and through the objective, and transmitted towards the ocular. 

 This, it is true, happens only when a lens is used dry (i.e., the 

 front surface in contact with air,) in which case rays inclined to 

 the axis at angles up to 87^'' actually do enter a well constructed 

 immersion lens. This angle, however, diminishes to about 48*>,* 

 when the lens is used wet, that is when water is dropped between 

 lens and covering glass as in the ordinary practice. This last 

 named angle is nevertheless of far higher amount than any angle of 

 aperture in the lens system of a telescope, or photograph camera, 

 because with such oblique incidence, the spherical aberration, even 

 in the carefully calculated and accurately executed lenses of these 

 instruments wouldbe simply intolerable. Why then, notwithstanding 

 this, is the large incident cone of light in the microscope more 

 advantageous than a narrow one of more intense light which would 

 deliver an equal absolute quantity ? The answer hitherto given to 

 this question appears to me unsatisfactory. For the so-called 

 *' penetration " (i.e., the power of delineating by light and shadow 

 and so rendering visible to the eye particles whose refractive quality 

 differs but slightly from that of the matter surrounding them,) 

 depends solely upon the proportion of the aperture of illuminating 

 cone, to that of the cone passing from points of the object into the 

 lens. Sufficient delineating shadow can only be got by narrowing 

 the aperture of the illuminating cone, and a comparatively large 

 cone can only be applied beneath the object when the cones of light 

 passing from it into the objective are also large. 



Now there does, in point of fact, exist in the microscope, a 

 special cause wl: ich under the conditions here given produces a far 

 greater aberration of rays from the focal plane than is occasioned by 

 spherical and chromatic aberration, and which makes itself most felt 

 just when the cones of incident light are smallest. This cause is 

 diffraction. 



* These figures it must be borne in mind, denote in each case the angle 

 included between outermost incident ray and axis of instrument, that is 

 half ihe, so-called " angle of aperture''' 



