198 
Gelehrte Anzeigen. Vols. XL VI. and XLVII. 4to . — From the 
same. 
Uber die geschichtlichen Vorstufen du neuern Rechtsphilosophie. 
Yon Prof. Dr Carl Prantl. 4to . — From the same. 
Uber Neuaufgefundene Dichtungen. Francesco Petrarca’s. Yon 
Prof. Dr G. M. Thomas. 4to . — From the same. 
IJber Johannes Muller und sein Yerhaltnis zum jetzigen. Standt- 
punkt der Physiologie. Yon Dr Th. L. W. Bischoff. 4to. — 
From the same. 
Catalogus Codicum Manu Scriptorum Bibliothecae Regise Mona- 
censis. Tomus YII. 8vo . — ■ From the same. 
Lighthouse Illumination : being a Description of the Holophotal 
system, and of Azimuthal-condensing, and Apparent Lights, 
with other Improvements. By Thomas Stevenson, F.R.S. E. 
8vo . — From the Author. 
Monday , 4 th April 1859. 
Professor CHPISTXSON, Vice-President in the Chair. 
The following Communications were read : — 
1. On the Gradual production of Luminous Impressions on 
the Eye. Part II. By Professor Swan. 
The author, in a paper communicated to the Royal Society of 
Edinburgh in 1849,* described a method of observation by which he 
had succeeded in measuring the brightness of visual impressions of 
short duration. This consisted in causing a disc, with a sector of 
a known angle cut in it, to revolve with a known uniform velocity 
between the eye and a luminous object. At each revolution of the 
disc a flash is seen. The time during which the light has acted on 
the eye is easily computed from the angle of the sector and velocity 
of the disc ; and the brightness of the flash is ascertained by photo- 
metric arrangements. By this method the brightness of impres- 
sions formed on the eye by light acting for short intervals of time 
varying from T to *001 of a second was ascertained with results 
which have been described in the paper already referred to. 
* Edinburgh Transactions, vol. xvi , p. 581. 
