as an experimentum crucis as to the nature of Light. 383 
trical on each side of it, although the whole appeared misty 
and without that darkness in the intervals which I had found 
in the previous experiments. This continued whatever pains 
I took to keep every part of the apparatus in adjustment. 
The dullness and mistiness of the latter phsenomenon led me 
to conclude that there could be no doubt but the former was 
the normal result. I however tried the experiment again 
with every care in the former circumstances, and found the 
same result as formerly. 
In the Number of Phil. Mag. for April, 1833, p. 279, I 
stated, ‘‘ The result of considerable experience with me is, 
that it may be seen both black and white, though with me it 
has much oftener been the former, especially when the bands 
have been well defined,” &c. &c. 
In the November Number for 1833, p. 342, I said, “ I 
shall consider it extremely important to determine whether, 
when an achromatic lens of short focus is used to form the 
luminous point, the central band of direct interference given 
by two mirrors is black, as it has appeared to me, and to 
several friends to whom I have shown it, when adequately 
tried with a common lens,” &c., &c. 
I have in the papers from which the above extracts are 
taken, publicly stated my difficulties without reserve, and 
my ideas of the requirements for a decisive mode of expe- 
rimenting. 
A mode of experimenting has been adopted, (where or 
with whom the discovery originated I do not know,) which 
is stated to give the central band white. It is this: the 
image of the sun formed in the focus of a lens is made 
to fall on a small aperture in a thin plate of metal ; the 
light passing through the aperture falls on the two mirrors, 
and the aperture thus illuminated is called the luminous 
origin. I am informed, that when the rays falling on the 
two mirrors are those which pass obliquely through the 
aperture, then the central band is most distinctly a white 
one; and it has been argued that this must be the normal 
way of trying the experiment, inasmuch as the rays of all 
colours will have accurately a common origin on the edge 
of the aperture. 
It has been with me a subject of frequent study to find 
out a method of trying the experiment which must give a 
result not to be disputed; and I feel confident the appa- 
ratus described in this paper fulfils every desideratum. 
The points kept in view have been to employ only such 
parts as are essential in the simplest form of the experi- 
ment; and thus in dispensing with the mirror which is 
