744 EAEL OE EOSSE ON THE CONSTEHCTION OE SPECULA OE 6-EEET APEETUEE, 
measure + or — this quantity will be the distance from the horizon, provided the tele- 
scope is on the meridian. When the measures are not taken on the meridian, but a 
little before or after it, there will be an error in all positions except at the equator. 
There mil be an error owing to two causes, one the error of the zero, the level having 
been read off the meridian ; the other the error in the reading of the position-circle, 
owing to the action of the universal joint which carries the telescope. The transverse 
axis of this joint restrains the movement of the telescope round the line of collima- 
tion as it approaches or recedes from the meridian ; and consequently the plane of the 
position-circle, except at the equator, does not pass through the pole. The sum of the 
errors in each case could easily have been computed and allowed for in the reductions, 
had the distance from the meridian been taken simultaneously with the measurements, 
but this would have taken much time. 
The measurements in the Dumb-bell accord pretty closely with Struve’s, and may, I 
think, be taken as a fair average of the work. 
As to H. 1622, the comparison of Struve’s measures with those of the two Stonets 
will give more than a probable amount of error at 42° N.P.D, because the stars are 
numerous, and some measures therefore were taken at a considerable distance from the 
meridian. 
I have not seen the Dumb-bell since Struve’s letter, having been from home when it 
was within reach ; and no attempt has been made to ascertain the largest number of 
stars visible in it. No stars have been inserted in the sketch which have not been 
measui’ed : very many more were distinctly seen. The number of stars visible in this 
nebula depends even more upon magnifying power and distinctness than aperture ; high 
powers obliterate the faint nebulous details. 
The only additional information as to the limits of the nebula which has been obtained 
since Mr. B. Stoxey’s drawing was made is contained in the following entry : — August 
29th, 1854, observed by Dr. Eobinson and Mr. J. Stojstey. Mr. Storey says, “Both 
Dr. Eobirsor and I agreed that the band of faint nebulosity extended further down 
than in my brother’s drawing. My brother and I had formed the same opinion on a 
previous occasion.” 
In the observations a silver speculum is sometimes mentioned ; we have employed 
silver occasionally for the second reflexion in this way. First, a thin deposit on glass 
by Liebig’s process. This, even when fresh, reflects but little more light than speculum- 
metal. Second, a thicJc deposit on glass, by the grape-sugar or tartaric-acid process, 
transferred to brass by a thin film of shell-lac : this reflects much more light ; but the 
manipulation is rather difficult, and the surface is not very durable. Third, a surface 
of standard silver, polished by mechanical means. Fourth, parallel glass, silvered by the 
grape-sugar process ; this of course is durable, but very inferior to the uncovered silver 
in light and in definition. These substitutes for speculum-metal have only been occa- 
sionally used, and for special purposes. 
