174 Mr. A. W. Porter on the Diffraction-Lmages 
magnetism we might expect that these would remove orienta- 
tion. But it is very difficult to draw any sure conclusion 
from such experiments, for the first is liable to disturb the 
delicate balance and perhaps change the surfaces of contact, 
whereas the second if carried far is sure to produce abundant 
oxide-film troubles. No conclusion could be drawn from the 
experiments made by these agencies. It is, however, very 
likely that recoherence dies away on account of the shock 
when the surfaces are brought repeatedly together. 
(6) The action of radium was tried without definite 
result. 
To summarize, Coherence can be best explained by the 
fusion theory. Recoherence is due to residual coherence, the 
clinging of the surfaces being apparently an adhesion of the 
particles (somewhat after the theory of Auerbach). It is 
hard to see how recoherence can be explained, except by some 
change in or orientation of surface particles occurring in the 
violent commotion of coherence. 
XIX. On the Diffraction Images formed by a Plane Diffraction- 
Grating. By ALFRED W. Porter, B.Sc., Fellow of, and 
Assistant Professor of Physics in, University College, 
London*. 
hare following problems all relate to the simplest kind of 
diffraction-grating consisting of alternate transparent 
and opaque strips, and the incident light is supposed to be 
normal. The intensity of the image formed at the focus of 
the observing telescope (adjusted for infinity) may be written, 
as usual, 
i. ee ew SI nv 
y ot ae ae = , 
Uu sin VU 
where n is the number of transparent strips , 
vie . TT . 
u=—rsnd, v=—(r+s) sin # 
Xr 2 rn ( ) } 
6 = direction of observation, and r and s are widths of trans- 
parent and opaque strips respectively. 
ProsBLeM A.— The criteria for positions of maxima and 
minima. 
These positions are usually found by equating to zero the 
first differential coefficient of F2 with regard to v. This 
* Communicated by the Author. Read at the University College 
Mathematical Society, March 1904. 
