whatever which is competent to exhibit as urious—or rather a 
18 A. C. Twining on Occultations by the Moon and Planets. 
which may be occulted—they being generally individuals of the 
lower magnitudes that would otherwise remain undistinguished 
among the multitudes of their class. Moreover, such a planet- 
occultation, although but one of a century, ne be more 
worthy of attention than any ordinary transit of Venu 
Mars, on account of its periodical presentation in ths dark 
hemisphere for six months together,—its slow motions back and =| 
forward during that period, “and its’ favorable magnitude of = 7 
disc, may not pr ipsen pe obtain the preference, even over Venus 
in this mode of employment,—notwithstanding the far greater — 
number of occultations, in a given time, by the latter. These 4 
remarks lead to the main topic of our enquiry, viz:— 
The means of observing a small star in contact with an illuminated 
dise, 
The object in this kind of aheePiaaon not being distinctness 
of visible form or color but the mere disappearance and rea 
pearance of stellar light, it is immaterial whether that hee pre 
sents itself asa brilliant point or as a mere round nebu 
The spurious | dises of several orders even of the fitter dare are 
Mie as will more definitely appear in the close of this paper— 
that a given Shellie of light augments in Deteep ee by ex- 
over an increased area or surface up to a certain degree 
of diffuseness, havond which the pemepaogyt or "effect upon : the 
aa of vision rapidly diminishes. It is possible therefore by 
_ increase of magnifying power to pass the maximum of visibility 
for a fainter illumination long before that maximum is reached. 
for a stronger. Not only so, but after the last named maximum 
is passed, the brighter pests (according to the law here- 
after deduced) fades, in its effect, much less rapidly than the 
weaker. In general therefore it may be presumed that any star 
riti 
__ focal disc in the telescope (a dise expan yy irregul: 
_ refraction) that exceeds very greatly in ‘{llumination the 
ary disc in the same field is also competent to exhibit visi 3 
phenomena of disappearance = reappearance of the star 
nd the : i t with the bright limb of 
