Capacihj of the Mieroscope. By Frof. Eelmlioltz. 31 
so sensitive as the first. But in all cases the best accommodation 
of the eye must be carefully maintained, otherwise the act of 
passing a card, pierced with an aperture, before the eye may, when 
there is imperfect accommodation, actually improve vision by dimi- 
nishing the dispersion. 
The theory of diffraction of rays in the microscope leads, as will 
be shown in the following pages, to the conclusion that any single 
point of light in the object must, when viewed through the 
microscope, appear exactly as if an actual luminous point, situate 
in the image of the object, were observed through an aperture 
corresponding in size and position to the ocular images (at 
the so-called eye spot) of the respective narrowest diaphragm 
aperture. 
Hence it follows, firstly, that diffraction phenomena must be 
visible when the ocular image has a diameter less than 1 "89 mm., 
and that the size of the dispersion circle, caused by diffraction, 
must increase in inverse proportion to the diameter of this ocular 
aperture, consequently in direct proportion to the amphfication, 
supposing that the incident light from each point in the object 
remains unchanged. Under such circumstances then, the image will 
not, even with higher amplifications, suffer further loss of sharpness 
of outline from diffraction, inasmuch as the dispersion circles 
preserve, throughout, the same relation to the apparent magnitude 
of the object. On the other hand, the deterioration arising from 
diminished brightness and multiplication of darker entoptic shadows, 
must increase with the amplification. From this it follows, 
therefore, that, as a general rule, that amount of amplification 
will show most detail by which the minutest points that are visible 
at all in the image, shall be presented under the most suitable 
visual angle, namely, somewhat larger than that at which an 
observer can distinguish the minutest objects visible to him under 
any circumstances. 
Calculated by the equation before mentioned, the diameter 
(1 • 89 mm.) of the area of light rays entering the pupil, when the 
light incident on the objective (in air) spreads out to nearly 180°, 
corresponds to an amphfication of 264 l. For objectives with less 
aperture the amplification must be set down at a lower figure. In 
^ • H. V. Mohl's handbook of the microscope it is stated that 
amplifications varying between 300 and 400 allow most detail to 
be seen, whilst Harting, speaking of more recent instruments with 
large angular aperture, found amphfications of 430 to 450 most 
serviceable. 
If now it be required to determine the magnitude of the minutest 
recognizable object as a standard by which to measure the accuracy 
of the microscopic image, we must not take for our unit the 
measured diameter of such obiects as bright single spots or lines on 
VOL. XVI. D 
