Capacity of the 3Iicroscope. Bij Prof. HehnJioUz. 35 
of the objective. The diffraction fringes caused by the use of a 
very deep ocular remained uncorrected. More recently I have 
convinced myself by fresh trials made with larger lenses, that such 
a procedure is useless. When a good achromatic lens of about 18 
inches focus is so placed as to show a sharp image of the source of 
light (as in this case a bright sky cloud) upon the surface of a 
system of hnes scratched on glass, the images of many separate 
luminous points will be thrown upon the variously transparent 
clefts of this grating, and it might be supposed that the interference 
of rays which had passed through neighbouring clefts would cease. 
If, however, we look through the grating towards the lens, and place 
before the lens pieces of card pierced with fine slits, we see with 
the naked eye just the same diffraction fringes, as well at these slits 
as at the outer edges of the cards, as would be seen if the lens were 
removed, or the grating set out of focus. 
Instead of the lines, I then made trial of two fine linear slits 
cut in cardboard, with an interspace of about 1 mm., and through 
which I could see with the naked eye a system of very fine inter- 
ference lines belonging to the diffraction image of another slit 
which was cut with the lines at a very small acute angle, sufiiciently 
narrow to produce the interference lines at the point of this angle. 
But these did not disappear when I threw an optical image of the 
incident light on the plane of the double (parallel) slit. In this 
experiment not the slightest suspicion could be entertained that 
chromatic or spherical aberration had dispersed the rays over an 
interspace of 1 mm. width. The only explanation I can offer is, 
that the light from the lens which passed through the acute angle 
of the slit serving here as object, suffers so strong a diffraction that 
it subsequently reaches the two openings of the doubly- sKt card 
with a corresponding wave-phase and therefore sends interfering 
bundles through both openings. In order to be able to see the 
interference -lines, it is necessary that their minima shall appear 
at a wider distance from each other than the width of the lines of 
which they are images, and when this condition is fulfilled theory 
does in fact show that the central clear portion of the diffraction 
figure of the simple slit forms a line of light which is broader than 
the distance between the two slits of the doubly-slit card. 
Similar relations take place (although more difficult to subject 
to calculation) when the fine edge of a dark screen is used as the 
object. It is known that from such an edge, bundles of interrupted 
rays (in linear formation) likewise bend . themselves into the dark 
field, which have corresponding phases of movement, and so when 
bent by a second screen can exhibit regular interference. That 
the resultant effect cannot become nil appears clearly from the 
fact that the effect of a bright line may be represented as the 
product of the action of two endless half-planes bounded by straight 
