Progress of Astronomy during the Year 1879. 453 
equations of condition, of which every observation furnishes one 
of the form 
1 
d—~(D+dD)+i 
Y fe 
or ¢ pa ass 
ft? “Le 
ed 
where D is an approximate value (17”50), and r the distance of 
Venus from the Earth. Herr Hartwig has, in addition to his 
own, discussed all published observations by other astronomers 
(many of them reduced anew), and the result is that nearly all 
measures are subject to rather large constant errors, as the 
following table shows. 
A,.——-HELIOMETER. 
ib Prob. Error. Aperture. 
ur Mr 
mm. 
1820 Brandes —1:096 +0°134 76 
1847-49 Wichmann —0-436 — 158 
1857 Winnecke —1:088 — 162 
1861-63 Main — 1-182 091 190 
1876-77 Hartwig S250 057 76 
B.--DovuBLE IMAGE MICROMETER (AIRY). 
i. Prob. Error. Aperture. 
fi as mm. 
1862-65 Kaiser —0°772 +0042 189 
1840-52 Main —0:50? =s 170 
1873 J. Plummer —0°546? — 152 
C.—Rocnon’s MIcROMETER. 
i. Prob. Error. Aperture. 
a uM mm. 
1810-15 Arago —1:519 +0105 162 
D.--WIRE MICROMETER. 
L Prob. Error. Aperture. 
tf di mm. 
1833-36 Midler +0°651 +0073 97 
F +0:116 0-245 
1854-57 J. Schmidt { Gate floss 
1860-63 Midler —0:254 — 244 
1888-39 Galle — 0°86 = 244 
1871 Vogel — 2°56 = 293°5 
The negative sign of the constant error for all double image 
observations is very remarkable, and it appears that telescopes 
of all apertures make the diameter as measured in daylight too 
small, while there does not seem to be any law depending on the 
aperture. In order to try whether a darker background would 
make the diameter turn out greater, observations were made 
in twilight on five evenings in March, 1879, and the re- 
sulting diameter was 0”°41 greater than when measured in full 
daylight. The greater sharpness and clearness of the image on 
; * Powers 120 and 90 respectively. 
