[ 49 ]. 



IX.— OBSEEVATION OP THE TRANSIT OF VEJ^US, 1882, DE- 

 CEMBER 6, MADE AT THE ARMAGH OBSERVATORY. 

 By J. L. E. DREYER, Ph. D. 



[Eead, December 18, 1882.] 



On the morning of the 6th of December the sky was quite over- 

 cast, and snow fell for about an hour between nine and ten o'clock. 

 About noon it began to clear, and with the exception of a few light 

 clouds passing now and then, the sky continued perfectly clear 

 until about three o'clock, when the sun was again hidden by 

 clouds. 



For observing the transit I employed the 15-inch reflector, 

 which is equatoreally mounted in the east tower of the Armagh 

 Observatory. The instrument can be used either in the Casse- 

 grain or in the Newtonian form : I preferred the latter, and had 

 a plane unsilvered mirror substituted for the usual metallic flat 

 mirror. The negative eye-piece (power 140) was furnished by 

 Mr. Grrubb, with a wedge of neutral-tinted glass, which permitted 

 the observer to diminish the intensity of the solar heat so as to 

 make it pleasant to the eye without rendering the mottled appear- 

 ance of the . photosphere indistinct. The eye-piece was carefully 

 focussed on the solar limb, and on several faculse, and with the 

 diminished aperture of seven inches both the limb and the mottled 

 surface of the sun were extremely well seen. 



I tried to observe the external contact of Yenus with an aper- 

 ture of eleven inches, but the definition was far from good, the 

 limb ' boiling ' violently. At 1^ 36°^ 14^, local mean time, I was 

 certain that the outer contact had taken place. Having exchanged 

 the diaphragm used with one of seven inches aperture, the defini- 

 tion was much improved, and before the second, or internal contact 

 occurred, it became almost all that could be desired. At 1^ 49"^ 31^ 

 the whole circumference of Yenus was visible, that part of the 

 planet which had not yet entered on the sun being projected on 

 the bright sky adjoining the solar limb. At 1^ 54"^ 49'-6 Yenus 

 was completely on the sun; but a faint, narrow shade-like band 



SCIEN. PROC, K.D.S. — VOL. IT. W- !• ' E 



