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TRANSACTIONS OF THE SECTIONS. 5 
_- On attempts to explain the apparent projection of a Star on the Moon. 
By the Rev. Professor PowEti, V.P.B.A. 
Some remarks having been brought forward at the last meeting* relative to the sin- 
gular phenomenon above named, in which “ diffraction” was referred to as at least 
in a general sense likely to afford an explanation, the author of this communication 
conceived that some observations he has made might have a bearing on the question. 
“< Diffraction” has often been appealed to in cases apparently of the same class, 
but in the more strict and limited sense of the term it cannot apply, since both the 
conditions and the resulting phenomena appear essentially different. 
The phenomena properly ascribed to “ diffraction” exhibit fringes, and suppose the 
edge of the intercepting body to be within the area of the rays. 
But there are some effects of a concomitant kind which have been less attended to. 
One of the most remarkable of these is that described by Newton (Opt. bk. iii. pt. 1. 
obs. 5, 6, 7), in which the light admitted through a hole a quarter of an inch in dia- 
meter, falling on the edge of an opake body, besides the phenomena since called 
“diffraction,” gave rise to long streaks or “trains” of light darting into the shadow 
perpendicular to the edge and shown on a screen; or, when the eye was substituted, 
producing a /uminous line running along the edge, between it and the first fringe. 
The author has repeated this experiment in a different manner, and though in the 
original experiment the edge is within the area of the rays, yet a part of the same phe- 
nomenon (viz. the line of light along the edge) is seen, even when the edge is beyond 
the rays, by the naked eye, or with a telescope. 
When the origin of light is reduced to a mere point (as by using the sun’s rays re- 
flected from a very small globule of mercury) and the rays are wholly intercepted by 
a small circular opake disc at the distance of about 2 inches, so that both the luminous 
point and the disc may be seen at once in focus by a small telescope about 12 feet dis- 
tant, the bright line is reduced to a luminous patch on the edge of the disc at the part 
nearest the luminous point which appeared to extend to a small distance inwards, and 
then the rays converging crossed and diverged again faintly. This might possibly be 
regarded as affording some experimental imitation of the case of the star: the origin 
is not an absolute point; but if it were, the patch of light on the disc might appear 
like a projection of its image. 
Another explanation has been proposed of the phenomenon of projection, on the 
principle that owing to aberration, the star being seen out of its true place, a screen 
placed in its érue direction, as the moon, would exhibit the star projected on its disc 
(Royal Astron. Soc. Reports, vol. vi. p. 246) ; and taking into account the proper mo- 
tions of the stars, this would explain the appearance of the phenomenon in one instance 
_ and not in another, on the supposition that those proper motions are in opposite direc- 
tions in the two instances. But this will not apply in the very instance to which re- 
ference has been made, of the two stars 119 and 120 Tauri, which have proper motions 
both in the same direction ; also, the principle of this explanation is rendered ques- 
tionable altogether from what has been lately suggested by Prof. Challis on the theory 
of aberration. 
The whole subject is perhaps not yet ripe for explanation, since the first astro- 
nomers are so much at variance as to the facts, the appearance having been frequently 
seen by one observer and not by another; while it is believed by some to occur or 
not, according as the attention is directed to the moon or to the star; which, if true, 
would seem to point to some ocular cause. Hence a further accumulation of in- 
stances is much wanted, any statements of which the author of this paper would be 
thankful to receive addressed to him at Oxford. 
On Elliptic Polarization. By Mr. Date. 
The paper which I have to read to the Section relates to some new observed facts 
in the subject of elliptic polarization, which appear to point out the physical element 
on which depends the different action of metals on light, as compared with transparent 
substances in general. They have already been communicated to the Ashmolean 
_ Society at Oxford, but I have been induced to bring them forward at present, with a 
* Brit. Assoc. Report, 1845, Sect. Proc. p. 5. 
