The Observations, made upon these proposals.
I Suppose the design of the Proposer of these Experiments
is, to have their events expressed, with such observations
as may occur concerning them. 1. Touching the first, I have
observ'd, that the Solar image falling on a paper placed at the focus of the Lens, was by the interposed Prism drawn out in
length proportional to the Prisms reflexion or distance from
that focus. And the chief observable here, which I remem
ber, was, that the Streight edges of the oblong image were
distincter than they would have been without the Lens.
Considering that the rays coming from the Planet Venus are
much less inclined one to another, than those, which come
from the opposite parts of the Suns disque; I once tryed an
experiment or two with her light. And to make it sufficient
ly strong, I found it necessary to collect it first by a broad lens,
and then interposing a Prism between the lens and its focus at
such distance, that all the light might pass through the Prism;
I found the focus, which before appeared like a lucid point, to
be drawn out into a long splendid line by the Prisms reflexion.
I have sometimes designed to try, how a fixt Star, seen through
a long Telescope, would appear by interposing a Prism be
tween the Telescope and my eye. But by the appearance of Venus, viewed with my naked eye through a Prism, I presage
the event.
2. Concerning the second experiment, I have occasionally ob
served, that by covering both ends of the Prism with Paper at
several distances from the midle, the breadth of the Solar
image will be increased or diminished as much, as is the aper
the aperture be augmented on all sides, the image on all sides
will be so much and no more augmented.
3. Of the third experiment I have occasion to speak in my
answer to another person; where you'l find the effects of two
Prisms in all cross positions of one to another described. But
if one Prism alone be turned about, the coloured image
will only be translated from place to place, describing a cir
cle or some other Conick Section on the wall, on which it is
projected, without suffering any alteration in its shape, unless
such as may arise from the obliquity of the wall or casual change
of the Prisms obliquity to the Suns rays.
4. The effect of the fourth experiment I have already insi
nuated telling you (in pag. 3076 of the Transactions) that Light,
passing through parts of the Prism of divers thicknesses, did
still exhibit the same Phænomena.
Note, that the long axes of the two Prisms in the experiment
described in the said pag. 3076 of the Transactions, were paral
lel one to another. And for the rest of their position, you will
by this Scheme;
where let EG de
sign the window;
F the hole in it,
through which the
light arrives at the
Prisms; ABC the first Prism, which
refracts the light
towards PT, paint
ing there the co
lour in an oblong
form; and second Prism, which refracts back again the
rays to Q where the long image PT is contracted into a round
one.
The plane
the rays may be equally refracted contrary ways in both Prisms.
And the Prisms must be placed very near to one another; For
light before its incidence on the second Prism, those colours
will not be destroyed by the contrary refractions of that Prism.
These things being observed, the round image Q will appear
of the same bigness, which it doth when both the Prisms are
taken away, that the light may pass directly towards Q from the
hole without any refraction at all. And its diameter will equal
the breadth of the long image PT, if those images be equally
distant from the Prisms.
If an accurate consideration of these refractions be designed,
it is convenient, that a Lens be placed in the hole F, or immedi
ately after the Prisms, so that its focus be at the image Q or PT.
For, thereby the Perimeter of the image Q and the straight sides
of the image PT will become much better defined than other
wise.