as an experimentum crucis as to the nature of Light, 387 
racy is not needed in lixing the cylinder, as its use is merely 
to afford the means of following the sun’s daily motion and 
still to leave the room dark. 
As the apparatus is here drawn, it will be advisable to have 
the mirrors ml oi speculum metal, for the angle of incidence 
of the light is too small for glass mirrors to give very bright 
phaenomena; although I have seen them of the same charac- 
ter as with the mirrors of speculum metal. The mirror k I 
have used of one-fourth of an inch focus. 
All the mirrors should be of metal which is not porous, 
and of the highest polish, so that their surface is not visible 
when they are placed a short distance from the flame of a 
candle ; for it is probable that numerous small pores in the 
metal, or the fine lines which constitute a second-rate polish, 
would produce diffraction sufficient to invalidate the experi- 
ment. As such mirrors are not everywhere to be had, I 
should, in such cases, recommend, as a substitute, that a good 
lens, of short focus, well centred, be placed in the aperture 
e f and that a pair of glass mirrors blackened at their second 
surface, be placed on the arm at the other end h ; the incidence 
would then be sufficiently oblique to afford an intense reflexion. 
Ordinary plate glass, or good pieces of window-glass, will 
show the bands very well; but those who wish to test the 
colour of the central band, should have their mirrors, as well 
as their lens, of good workmanship. The obtuse-angled 
prisms which m.ay be obtained in the shops, are very conve- 
nient for observing the bands popularly, but are not to be 
depended upon as test apparatus. 
Those who use the two mirrors for the first time will find 
some care necessary in fixing them with their contiguous 
edges in contact, for if the edge of one be raised any appre- 
ciable distance above that of the other, the bands are not 
produced. With glass mirrors, blackened at their second 
surface by sealing-wax melted over them, it is convenient to 
cement them together with the same material along their line 
of contact. If they are placed in position whilst the sealing- 
wax is soft, and then retained in their position until cool, they 
will remain ready adjusted to be used at any time. 
I have been the more minute and popular in explaining the 
theoretical bearings, as well as the experimental details, from 
the consideration that this interesting and important experi- 
ment has very singularly been but little noticed in our more 
descriptive optical treatises ; and also from the hope that I 
may by this essay induce others to try it, who have a sincere 
wish to form a correct opinion on the nature of light. 
