178 Scientific Intelligence, 
ger curve than that of the lunar crescent, when the moon is about 
two days old. The difference of longitude between the Sun 
and Venus at that time was about 2° 19'. This is the nearest 
position to the point of conjunction in which Venus has been 
seen in any former recorded observation ; and, from the degrees > 
of distinctness with which the planet appeared, there is little 
doubt that its crescent would have been perceptible twelve hours 
before, had it been then above our horizon. From this obser- 
vation the following conclusion is deduced, viz. that “ from the 
time that this planet ceases to be visible prior to its inferior con- 
junction, on account of the smallness of its crescent, and its 
proximity to the sun, to the moment when it may again be per- 
ceived in the day-time by an Equatorial Telescope, there 
elapses a period of only 2 days and 22 hours or, in other 
words, Venus can never be hid from our view about the time of 
her inferior conjunction, for a longer period than 70 hours. 
From observations formerly made on this planet about the time 
of its superior conjunction, by the same observer, it appears, 
that Venus can be hid only 6 days 8^ hours before,^ and the 
same time after that conjunction ; and, consequently, it will fol- 
low, that, during the space of 582 days, the time she takes in 
moving from one conjunction to a like conjunction again, she 
may be seen by means of an Equatorial Telescope, every clear 
day, without interruption, except during a period of 15 days 
and 15 hours. — Pertli^ \Wi March 1822. 
OPTICS.. 
13. Laws of Polarisationrin rectangular Plates of Glass . — 
If we call the distances of any point from the lines of no 
polarisation in rectangular plates of glass, crystallised either 
transiently or permanently by heat, the tint t at that point will 
be = T ^ T being the maximum tint between the black 
lines. In like manner, = V^ 4- « ^ V will express the square of 
the velocity of the extraordinary ray, V^ being that of the ordinary 
one, and a a co-efficient, varying mth the plate of glass. — D. B. 
14. Frauenhqfer’^s Experiments on the Prismatic Spectrum , — 
M. Frauenhofer of Benedictbaiern, near Munich, who has been 
long celebrated for the excellence of his achromatic telescopes, 
has made, a considerable time ago, some important and curious 
discoveries respecting the prismatic spectrum. He found, that 
