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Grusp— On a New Form of Equatorial Telescope. 65 
For protection against weather. The instrument is shown erected between two 
walls sufficiently low to allow it to sweep E. and W.; on these walls a wooden 
framework or roof travels, which can be rolled over the instrument when the tube 
is placed in a horizontal position. 
The instrument commands the heavens from E. to W., and from the south hori- 
zon to about 20° beyond the zenith. 
In the plate no tube or connexion is shown between the equatorial and the eye- 
piece—probably in most cases it would be desirable to have a connecting tube; 
otherwise the images would revolve in the micrometer, and the zero of the position 
angle would be continually varying. This tube would preferably be made open- 
work or latticed, to avoid the currents of air which would be liable to be set up in 
a closed tube, the ends of which would be at different temperatures. 
A form of instrument which partially fulfils the conditions I desired to attain to 
is figured in Natwre of 8th November, 1883, and one of small size has already been 
erected in the Paris Observatory. As regards this instrument, I would observe 
that it possibly possesses an advantage over my form in being absolutely univer- 
sal; it could, at least, be made so. 
This I considered of little consequence, as almost the whole of the work suitable 
for such a large instrument can be commanded by the form above described. No 
doubt, universality, if it could be secured without corresponding disadvantages, 
would be a certain gain; but I believe that in the endeayour to obtain it, and at the 
same time to make the instrument one of precision,- enormous sacrifices have been 
made ; and I look upon the attempt to combine these two qualities in a single tele- 
scope as at once vain and unprofitable. 
I think I may venture to say that it is the universal opinion of astronomers 
that no large instruments with two axes, one of which varies in its direction to the 
horizon, have ever been made instruments of precision. Nor even if they were 
would they be useful. Such instruments, so far as measuring is concerned, could 
only be used for measuring differences between the position of objects under exa- 
mination and some known object close by, whose place has already been deter- 
mined by instruments of precision, such as transit circles, &c. 
In attempting to fulfil the two conditions of universality and precision, the 
French instrument is supplied with two reflectors, one half way in the tube 
(which would about correspond with the one in mine), and the other outside 
the objective. 
Now the great difficulty about these instruments is, of course, the production 
of a perfectly plane surface on the mirror. If it were not for this, they would 
have been made and universally used long since. Some persons are at present 
even sceptical of the possibility of any optician producing a sufficiently accurate 
_plane surface. No doubt, at all events, exists that there is no piece of work 
which calls into play more fully the skill of the mechanician than the production 
