476 Scientific Proceedings, Royal Dublin Society. 
As most of these stars would be very troublesome to identify in 
the field of a photometer mounted on an ordinary stand they are 
observed in the meridian. The photometer consists of a hori- 
zontal telescope pointing to the west, and having two objectives. 
By means of two prisms mounted in front of the telescope the 
pole star is reflected into one object-glass, and the star to be 
measured into the other. The cones of light are made to coincide 
by a double-image prism, the extra images being cut off by an 
eye-stop. The star to be measured is thus seen in the same 
field with the pole star, under exactly the same conditions. To 
determine the relative transparency of the air at different 
altitudes, a list of a hundred circumpolar stars has been pre- 
pared, to be observed at both upper and lower culminations. 
Progressive changes in the light of the pole star may thus also 
be detected and eliminated. 
15. History of Astronomy ; Bibliography. 
A Persian MS. of Ulugh Beg’s catalogue of stars recently pre- 
sented to the R. Astronomical Society has been examined by 
Mr. Knobel (M.N. xxx., 337), and compared with Hyde’s transla- 
tion. A great many discrepancies have (as was also the case 
with Sufi) been found to arise from certain characters being 
mistaken for others by the transcriber, and Mr. Knobel was hereby 
led to examine those of Ibn Junis’s lunar eclipses, which Pro- 
fessor Newcomb had found to be irreconcilable with the 
computations, and he suggests explanations, founded on the 
assumption of similar errors having been made in copying a 
MS. The magnitudes of Ulugh Beg never having been pro- 
perly translated, Mr. Knobel gives a complete translation of 
the magnitudes as found in the MS. under examination. 
Professor Oudemans, of Utrecht, has communicated to the 
Astr. Nachr., No. 2277, some interesting researches as to who in 
reality has discovered the negative eyepiece, consisting of two 
plano-convex lenses. By searching Huyghens’ “ Dioptrica” he 
found only an eyepiece mentioned, consisting of two biconvex 
lenses, in which the proportions were f:d:f’ = 4:2:1*, while 
plano-convex lenses are not alluded to anywhere. Already in 
+ [Opera posthuma, Amsterdam, 1728, I., p. 140.] 
